[v2,3/3] sched/psi: allow unprivileged polling of N*2s period

  PSI offers 2 mechanisms to get information about a specific resource
pressure. One is reading from /proc/pressure/<resource>, which gives
average pressures aggregated every 2s. The other is creating a pollable
fd for a specific resource and cgroup.

The trigger creation requires CAP_SYS_RESOURCE, and gives the
possibility to pick specific time window and threshold, spawing an RT
thread to aggregate the data.

Systemd would like to provide containers the option to monitor pressure
on their own cgroup and sub-cgroups. For example, if systemd launches a
container that itself then launches services, the container should have
the ability to poll() for pressure in individual services. But neither
the container nor the services are privileged.

This patch implements a mechanism to allow unprivileged users to create
pressure triggers. The difference with privileged triggers creation is
that unprivileged ones must have a time window that's a multiple of 2s.
This is so that we can avoid unrestricted spawning of rt threads, and
use instead the same aggregation mechanism done for the averages, which
runs independently of any triggers.

Suggested-by: Johannes Weiner <hannes@cmpxchg.org>
Signed-off-by: Domenico Cerasuolo <cerasuolodomenico@gmail.com>
---
 Documentation/accounting/psi.rst |   4 ++
 include/linux/psi.h              |   2 +-
 include/linux/psi_types.h        |  11 +++-
 kernel/cgroup/cgroup.c           |   2 +-
 kernel/sched/psi.c               | 110 ++++++++++++++++++-------------
 5 files changed, 78 insertions(+), 51 deletions(-)
  

On Thu, Mar 23, 2023 at 11:33:50AM +0100, Domenico Cerasuolo wrote:
> @@ -151,6 +151,14 @@ struct psi_trigger {
>  
>  	/* Deferred event(s) from previous ratelimit window */
>  	bool pending_event;
> +
> +	/* Used to differentiate destruction action*/
> +	enum psi_aggregators aggregator;
> +};
> +
> +struct trigger_info {
> +	struct list_head triggers;
> +	u32 nr_triggers[NR_PSI_STATES - 1];
>  };
>  
>  struct psi_group {
> @@ -186,8 +194,7 @@ struct psi_group {
>  	struct mutex trigger_lock;
>  
>  	/* Configured polling triggers */
> -	struct list_head triggers;
> -	u32 nr_triggers[NR_PSI_STATES - 1];
> +	struct trigger_info trig_info[NR_PSI_AGGREGATORS];
>  	u32 poll_states;
>  	u64 poll_min_period;

Thanks for trying out this variant, but I think this is grouping up
unrelated things, and that makes the code more difficult to understand
and maintan.

The *only* thing that's shared between those two is the
update_triggers() part. trig_info[PSI_AVGS] doesn't use trigger_lock.
It also doesn't use poll_task, poll_wait, poll_wakeup, poll_scheduled,
poll_min_period, polling_next_update and polling_until. All these
things are specific to the rt polling thread.

The rename in the previous version is a bit churny, but it's justified
in order to keep unrelated things separate / make it obvious which
parts belong together, and who is reading and writing which fields.

So my vote would be on the previous version.
  

On Thu, Mar 23, 2023 at 8:09 AM Johannes Weiner <hannes@cmpxchg.org> wrote:
>
> On Thu, Mar 23, 2023 at 11:33:50AM +0100, Domenico Cerasuolo wrote:
> > @@ -151,6 +151,14 @@ struct psi_trigger {
> >
> >       /* Deferred event(s) from previous ratelimit window */
> >       bool pending_event;
> > +
> > +     /* Used to differentiate destruction action*/
> > +     enum psi_aggregators aggregator;
> > +};
> > +
> > +struct trigger_info {
> > +     struct list_head triggers;
> > +     u32 nr_triggers[NR_PSI_STATES - 1];
> >  };
> >
> >  struct psi_group {
> > @@ -186,8 +194,7 @@ struct psi_group {
> >       struct mutex trigger_lock;
> >
> >       /* Configured polling triggers */
> > -     struct list_head triggers;
> > -     u32 nr_triggers[NR_PSI_STATES - 1];
> > +     struct trigger_info trig_info[NR_PSI_AGGREGATORS];
> >       u32 poll_states;
> >       u64 poll_min_period;
>
> Thanks for trying out this variant, but I think this is grouping up
> unrelated things, and that makes the code more difficult to understand
> and maintan.
>
> The *only* thing that's shared between those two is the
> update_triggers() part. trig_info[PSI_AVGS] doesn't use trigger_lock.
> It also doesn't use poll_task, poll_wait, poll_wakeup, poll_scheduled,
> poll_min_period, polling_next_update and polling_until. All these
> things are specific to the rt polling thread.
>
> The rename in the previous version is a bit churny, but it's justified
> in order to keep unrelated things separate / make it obvious which
> parts belong together, and who is reading and writing which fields.
>
> So my vote would be on the previous version.

Hmm. Ok, but then I would suggest keeping RT trigger naming as is and
calling the new triggers based on averages as
avg_triggers/avg_nr_triggers/etc. This would limit the churn and since
we already have polling_total and avg_total, this naming would be
appropriate IMO. If we want to be even stricter, we could rename the
polling variables to poll_triggers/poll_nr_triggers/etc.  Some more
churn but then the names are very distinct.
  

On Thu, Mar 23, 2023 at 3:34 AM Domenico Cerasuolo
<cerasuolodomenico@gmail.com> wrote:
>
> PSI offers 2 mechanisms to get information about a specific resource
> pressure. One is reading from /proc/pressure/<resource>, which gives
> average pressures aggregated every 2s. The other is creating a pollable
> fd for a specific resource and cgroup.
>
> The trigger creation requires CAP_SYS_RESOURCE, and gives the
> possibility to pick specific time window and threshold, spawing an RT
> thread to aggregate the data.
>
> Systemd would like to provide containers the option to monitor pressure
> on their own cgroup and sub-cgroups. For example, if systemd launches a
> container that itself then launches services, the container should have
> the ability to poll() for pressure in individual services. But neither
> the container nor the services are privileged.
>
> This patch implements a mechanism to allow unprivileged users to create
> pressure triggers. The difference with privileged triggers creation is
> that unprivileged ones must have a time window that's a multiple of 2s.
> This is so that we can avoid unrestricted spawning of rt threads, and
> use instead the same aggregation mechanism done for the averages, which
> runs independently of any triggers.
>
> Suggested-by: Johannes Weiner <hannes@cmpxchg.org>
> Signed-off-by: Domenico Cerasuolo <cerasuolodomenico@gmail.com>
> ---
>  Documentation/accounting/psi.rst |   4 ++
>  include/linux/psi.h              |   2 +-
>  include/linux/psi_types.h        |  11 +++-
>  kernel/cgroup/cgroup.c           |   2 +-
>  kernel/sched/psi.c               | 110 ++++++++++++++++++-------------
>  5 files changed, 78 insertions(+), 51 deletions(-)
>
> diff --git a/Documentation/accounting/psi.rst b/Documentation/accounting/psi.rst
> index 5e40b3f437f9..df6062eb3abb 100644
> --- a/Documentation/accounting/psi.rst
> +++ b/Documentation/accounting/psi.rst
> @@ -105,6 +105,10 @@ prevent overly frequent polling. Max limit is chosen as a high enough number
>  after which monitors are most likely not needed and psi averages can be used
>  instead.
>
> +Unprivileged users can also create monitors, with the only limitation that the
> +window size must be a multiple of 2s, in order to prevent excessive resource
> +usage.
> +
>  When activated, psi monitor stays active for at least the duration of one
>  tracking window to avoid repeated activations/deactivations when system is
>  bouncing in and out of the stall state.
> diff --git a/include/linux/psi.h b/include/linux/psi.h
> index b029a847def1..ab26200c2803 100644
> --- a/include/linux/psi.h
> +++ b/include/linux/psi.h
> @@ -24,7 +24,7 @@ void psi_memstall_leave(unsigned long *flags);
>
>  int psi_show(struct seq_file *s, struct psi_group *group, enum psi_res res);
>  struct psi_trigger *psi_trigger_create(struct psi_group *group,
> -                       char *buf, enum psi_res res);
> +                       char *buf, enum psi_res res, struct file *file);
>  void psi_trigger_destroy(struct psi_trigger *t);
>
>  __poll_t psi_trigger_poll(void **trigger_ptr, struct file *file,
> diff --git a/include/linux/psi_types.h b/include/linux/psi_types.h
> index 1e0a0d7ace3a..eaee30f54670 100644
> --- a/include/linux/psi_types.h
> +++ b/include/linux/psi_types.h
> @@ -151,6 +151,14 @@ struct psi_trigger {
>
>         /* Deferred event(s) from previous ratelimit window */
>         bool pending_event;
> +
> +       /* Used to differentiate destruction action*/
> +       enum psi_aggregators aggregator;
> +};
> +
> +struct trigger_info {
> +       struct list_head triggers;
> +       u32 nr_triggers[NR_PSI_STATES - 1];
>  };
>
>  struct psi_group {
> @@ -186,8 +194,7 @@ struct psi_group {
>         struct mutex trigger_lock;
>
>         /* Configured polling triggers */
> -       struct list_head triggers;
> -       u32 nr_triggers[NR_PSI_STATES - 1];
> +       struct trigger_info trig_info[NR_PSI_AGGREGATORS];
>         u32 poll_states;
>         u64 poll_min_period;
>
> diff --git a/kernel/cgroup/cgroup.c b/kernel/cgroup/cgroup.c
> index 935e8121b21e..dead36969bba 100644
> --- a/kernel/cgroup/cgroup.c
> +++ b/kernel/cgroup/cgroup.c
> @@ -3761,7 +3761,7 @@ static ssize_t pressure_write(struct kernfs_open_file *of, char *buf,
>         }
>
>         psi = cgroup_psi(cgrp);
> -       new = psi_trigger_create(psi, buf, res);
> +       new = psi_trigger_create(psi, buf, res, of->file);
>         if (IS_ERR(new)) {
>                 cgroup_put(cgrp);
>                 return PTR_ERR(new);
> diff --git a/kernel/sched/psi.c b/kernel/sched/psi.c
> index 17d71ef07751..f15d92819fe5 100644
> --- a/kernel/sched/psi.c
> +++ b/kernel/sched/psi.c
> @@ -180,18 +180,20 @@ static void poll_timer_fn(struct timer_list *t);
>  static void group_init(struct psi_group *group)
>  {
>         int cpu;
> +       int i;
>
>         group->enabled = true;
>         for_each_possible_cpu(cpu)
>                 seqcount_init(&per_cpu_ptr(group->pcpu, cpu)->seq);
>         group->avg_last_update = sched_clock();
>         group->avg_next_update = group->avg_last_update + psi_period;
> -       INIT_DELAYED_WORK(&group->avgs_work, psi_avgs_work);
>         mutex_init(&group->avgs_lock);
>         /* Init trigger-related members */
>         atomic_set(&group->poll_scheduled, 0);
>         mutex_init(&group->trigger_lock);
> -       INIT_LIST_HEAD(&group->triggers);
> +       for (i = 0; i < NR_PSI_AGGREGATORS; i++)
> +               INIT_LIST_HEAD(&group->trig_info[i].triggers);
> +       INIT_DELAYED_WORK(&group->avgs_work, psi_avgs_work);
>         group->poll_min_period = U32_MAX;
>         group->polling_next_update = ULLONG_MAX;
>         init_waitqueue_head(&group->poll_wait);
> @@ -430,21 +432,24 @@ static u64 window_update(struct psi_window *win, u64 now, u64 value)
>         return growth;
>  }
>
> -static u64 update_triggers(struct psi_group *group, u64 now, bool *update_total)
> +static u64 update_triggers(struct psi_group *group, u64 now, bool *update_total,
> +                                                  enum psi_aggregators aggregator)
>  {
>         struct psi_trigger *t;
> -       u64 *total = group->total[PSI_POLL];
> +       u64 *total = group->total[aggregator];
> +       struct list_head *triggers = &group->trig_info[aggregator].triggers;
> +       u64 *aggregator_total = aggregator == PSI_AVGS ? group->avg_total : group->polling_total;
>         *update_total = false;
>
>         /*
>          * On subsequent updates, calculate growth deltas and let
>          * watchers know when their specified thresholds are exceeded.
>          */
> -       list_for_each_entry(t, &group->triggers, node) {
> +       list_for_each_entry(t, triggers, node) {
>                 u64 growth;
>                 bool new_stall;
>
> -               new_stall = group->polling_total[t->state] != total[t->state];
> +               new_stall = aggregator_total[t->state] != total[t->state];
>
>                 /* Check for stall activity or a previous threshold breach */
>                 if (!new_stall && !t->pending_event)
> @@ -545,6 +550,7 @@ static void psi_avgs_work(struct work_struct *work)
>  {
>         struct delayed_work *dwork;
>         struct psi_group *group;
> +       bool update_total;
>         u32 changed_states;
>         u64 now;
>
> @@ -563,8 +569,10 @@ static void psi_avgs_work(struct work_struct *work)
>          * Once restarted, we'll catch up the running averages in one
>          * go - see calc_avgs() and missed_periods.
>          */
> -       if (now >= group->avg_next_update)
> +       if (now >= group->avg_next_update) {
> +               update_triggers(group, now, &update_total, PSI_AVGS);
>                 group->avg_next_update = update_averages(group, now);
> +       }
>
>         if (changed_states & PSI_STATE_RESCHEDULE) {
>                 schedule_delayed_work(dwork, nsecs_to_jiffies(
> @@ -574,11 +582,11 @@ static void psi_avgs_work(struct work_struct *work)
>         mutex_unlock(&group->avgs_lock);
>  }
>
> -static void init_triggers(struct psi_group *group, u64 now)
> +static void init_poll_triggers(struct psi_group *group, u64 now)
>  {
>         struct psi_trigger *t;
>
> -       list_for_each_entry(t, &group->triggers, node)
> +       list_for_each_entry(t, &group->trig_info[PSI_POLL].triggers, node)
>                 window_reset(&t->win, now,
>                                 group->total[PSI_POLL][t->state], 0);
>         memcpy(group->polling_total, group->total[PSI_POLL],
> @@ -667,7 +675,7 @@ static void psi_poll_work(struct psi_group *group)
>         if (changed_states & group->poll_states) {
>                 /* Initialize trigger windows when entering polling mode */
>                 if (now > group->polling_until)
> -                       init_triggers(group, now);
> +                       init_poll_triggers(group, now);
>
>                 /*
>                  * Keep the monitor active for at least the duration of the
> @@ -684,7 +692,7 @@ static void psi_poll_work(struct psi_group *group)
>         }
>
>         if (now >= group->polling_next_update) {
> -               group->polling_next_update = update_triggers(group, now, &update_total);
> +               group->polling_next_update = update_triggers(group, now, &update_total, PSI_POLL);
>                 if (update_total)
>                         memcpy(group->polling_total, group->total[PSI_POLL],
>                                    sizeof(group->polling_total));
> @@ -1254,16 +1262,19 @@ int psi_show(struct seq_file *m, struct psi_group *group, enum psi_res res)
>  }
>
>  struct psi_trigger *psi_trigger_create(struct psi_group *group,
> -                       char *buf, enum psi_res res)
> +                       char *buf, enum psi_res res, struct file *file)
>  {
>         struct psi_trigger *t;
>         enum psi_states state;
>         u32 threshold_us;
> +       bool privileged;
>         u32 window_us;
>
>         if (static_branch_likely(&psi_disabled))
>                 return ERR_PTR(-EOPNOTSUPP);
>
> +       privileged = cap_raised(file->f_cred->cap_effective, CAP_SYS_RESOURCE);

I missed one detail here. We are moving the cap check from open() to
write(). That might break potential users which open the file from a
process with that cap and then pass that FD to an unprivileged process
to create the trigger by writing to that file. I'm not aware of any
use of such a pattern but it is possible there are such users.
With this change such users would have to delegate trigger creation to
the privileged process too and the received FD would be used only for
polling. IMHO that's a safer pattern because triggers are created by
the privileged process.

> +
>         if (sscanf(buf, "some %u %u", &threshold_us, &window_us) == 2)
>                 state = PSI_IO_SOME + res * 2;
>         else if (sscanf(buf, "full %u %u", &threshold_us, &window_us) == 2)
> @@ -1283,6 +1294,13 @@ struct psi_trigger *psi_trigger_create(struct psi_group *group,
>                 window_us > WINDOW_MAX_US)
>                 return ERR_PTR(-EINVAL);
>
> +       /*
> +        * Unprivileged users can only use 2s windows so that averages aggregation
> +        * work is used, and no RT threads need to be spawned.
> +        */
> +       if (!privileged && window_us % 2000000)
> +               return ERR_PTR(-EINVAL);
> +
>         /* Check threshold */
>         if (threshold_us == 0 || threshold_us > window_us)
>                 return ERR_PTR(-EINVAL);
> @@ -1302,10 +1320,11 @@ struct psi_trigger *psi_trigger_create(struct psi_group *group,
>         t->last_event_time = 0;
>         init_waitqueue_head(&t->event_wait);
>         t->pending_event = false;
> +       t->aggregator = privileged ? PSI_POLL : PSI_AVGS;
>
>         mutex_lock(&group->trigger_lock);
>
> -       if (!rcu_access_pointer(group->poll_task)) {
> +       if (privileged && !rcu_access_pointer(group->poll_task)) {
>                 struct task_struct *task;
>
>                 task = kthread_create(psi_poll_worker, group, "psimon");
> @@ -1319,12 +1338,14 @@ struct psi_trigger *psi_trigger_create(struct psi_group *group,
>                 rcu_assign_pointer(group->poll_task, task);
>         }
>
> -       list_add(&t->node, &group->triggers);
> -       group->poll_min_period = min(group->poll_min_period,
> -               div_u64(t->win.size, UPDATES_PER_WINDOW));
> -       group->nr_triggers[t->state]++;
> -       group->poll_states |= (1 << t->state);
> +       list_add(&t->node, &group->trig_info[t->aggregator].triggers);
> +       group->trig_info[t->aggregator].nr_triggers[t->state]++;
>
> +       if (privileged) {
> +               group->poll_min_period = min(group->poll_min_period,
> +                       div_u64(t->win.size, UPDATES_PER_WINDOW));
> +               group->poll_states |= (1 << t->state);
> +       }
>         mutex_unlock(&group->trigger_lock);
>
>         return t;
> @@ -1357,22 +1378,25 @@ void psi_trigger_destroy(struct psi_trigger *t)
>                 u64 period = ULLONG_MAX;
>
>                 list_del(&t->node);
> -               group->nr_triggers[t->state]--;
> -               if (!group->nr_triggers[t->state])
> -                       group->poll_states &= ~(1 << t->state);
> -               /* reset min update period for the remaining triggers */
> -               list_for_each_entry(tmp, &group->triggers, node)
> -                       period = min(period, div_u64(tmp->win.size,
> -                                       UPDATES_PER_WINDOW));
> -               group->poll_min_period = period;
> -               /* Destroy poll_task when the last trigger is destroyed */
> -               if (group->poll_states == 0) {
> -                       group->polling_until = 0;
> -                       task_to_destroy = rcu_dereference_protected(
> -                                       group->poll_task,
> -                                       lockdep_is_held(&group->trigger_lock));
> -                       rcu_assign_pointer(group->poll_task, NULL);
> -                       del_timer(&group->poll_timer);
> +               group->trig_info[t->aggregator].nr_triggers[t->state]--;
> +
> +               if (t->aggregator == PSI_POLL) {
> +                       if (!group->trig_info[t->aggregator].nr_triggers[t->state])
> +                               group->poll_states &= ~(1 << t->state);
> +                       /* reset min update period for the remaining triggers */
> +                       list_for_each_entry(tmp, &group->trig_info[t->aggregator].triggers, node)
> +                               period = min(period, div_u64(tmp->win.size,
> +                                               UPDATES_PER_WINDOW));
> +                       group->poll_min_period = period;
> +                       /* Destroy poll_task when the last trigger is destroyed */
> +                       if (group->poll_states == 0) {
> +                               group->polling_until = 0;
> +                               task_to_destroy = rcu_dereference_protected(
> +                                               group->poll_task,
> +                                               lockdep_is_held(&group->trigger_lock));
> +                               rcu_assign_pointer(group->poll_task, NULL);
> +                               del_timer(&group->poll_timer);
> +                       }

You could minimize the churn here. For (t->aggregator == PSI_POLL)
case we are doing some additional stuff, so instead of doing "if
(t->aggregator == PSI_POLL) { " and shifting the rest of the code you
could have "if (t->aggregator == PSI_AVGS) return;" and keep the
existing indent level.

>                 }
>         }
>
> @@ -1436,27 +1460,19 @@ static int psi_cpu_show(struct seq_file *m, void *v)
>         return psi_show(m, &psi_system, PSI_CPU);
>  }
>
> -static int psi_open(struct file *file, int (*psi_show)(struct seq_file *, void *))
> -{
> -       if (file->f_mode & FMODE_WRITE && !capable(CAP_SYS_RESOURCE))
> -               return -EPERM;
> -
> -       return single_open(file, psi_show, NULL);
> -}
> -
>  static int psi_io_open(struct inode *inode, struct file *file)
>  {
> -       return psi_open(file, psi_io_show);
> +       return single_open(file, psi_io_show, NULL);
>  }
>
>  static int psi_memory_open(struct inode *inode, struct file *file)
>  {
> -       return psi_open(file, psi_memory_show);
> +       return single_open(file, psi_memory_show, NULL);
>  }
>
>  static int psi_cpu_open(struct inode *inode, struct file *file)
>  {
> -       return psi_open(file, psi_cpu_show);
> +       return single_open(file, psi_cpu_show, NULL);
>  }
>
>  static ssize_t psi_write(struct file *file, const char __user *user_buf,
> @@ -1490,7 +1506,7 @@ static ssize_t psi_write(struct file *file, const char __user *user_buf,
>                 return -EBUSY;
>         }
>
> -       new = psi_trigger_create(&psi_system, buf, res);
> +       new = psi_trigger_create(&psi_system, buf, res, file);
>         if (IS_ERR(new)) {
>                 mutex_unlock(&seq->lock);
>                 return PTR_ERR(new);
> @@ -1570,7 +1586,7 @@ static int psi_irq_show(struct seq_file *m, void *v)
>
>  static int psi_irq_open(struct inode *inode, struct file *file)
>  {
> -       return psi_open(file, psi_irq_show);
> +       return single_open(file, psi_irq_show, NULL);
>  }
>
>  static ssize_t psi_irq_write(struct file *file, const char __user *user_buf,
> --
> 2.34.1
>
  

On Thu, Mar 23, 2023 at 09:43:16AM -0700, Suren Baghdasaryan wrote:
> On Thu, Mar 23, 2023 at 8:09 AM Johannes Weiner <hannes@cmpxchg.org> wrote:
> >
> > On Thu, Mar 23, 2023 at 11:33:50AM +0100, Domenico Cerasuolo wrote:
> > > @@ -151,6 +151,14 @@ struct psi_trigger {
> > >
> > >       /* Deferred event(s) from previous ratelimit window */
> > >       bool pending_event;
> > > +
> > > +     /* Used to differentiate destruction action*/
> > > +     enum psi_aggregators aggregator;
> > > +};
> > > +
> > > +struct trigger_info {
> > > +     struct list_head triggers;
> > > +     u32 nr_triggers[NR_PSI_STATES - 1];
> > >  };
> > >
> > >  struct psi_group {
> > > @@ -186,8 +194,7 @@ struct psi_group {
> > >       struct mutex trigger_lock;
> > >
> > >       /* Configured polling triggers */
> > > -     struct list_head triggers;
> > > -     u32 nr_triggers[NR_PSI_STATES - 1];
> > > +     struct trigger_info trig_info[NR_PSI_AGGREGATORS];
> > >       u32 poll_states;
> > >       u64 poll_min_period;
> >
> > Thanks for trying out this variant, but I think this is grouping up
> > unrelated things, and that makes the code more difficult to understand
> > and maintan.
> >
> > The *only* thing that's shared between those two is the
> > update_triggers() part. trig_info[PSI_AVGS] doesn't use trigger_lock.
> > It also doesn't use poll_task, poll_wait, poll_wakeup, poll_scheduled,
> > poll_min_period, polling_next_update and polling_until. All these
> > things are specific to the rt polling thread.
> >
> > The rename in the previous version is a bit churny, but it's justified
> > in order to keep unrelated things separate / make it obvious which
> > parts belong together, and who is reading and writing which fields.
> >
> > So my vote would be on the previous version.
> 
> Hmm. Ok, but then I would suggest keeping RT trigger naming as is and
> calling the new triggers based on averages as
> avg_triggers/avg_nr_triggers/etc. This would limit the churn and since
> we already have polling_total and avg_total, this naming would be
> appropriate IMO. If we want to be even stricter, we could rename the
> polling variables to poll_triggers/poll_nr_triggers/etc.  Some more
> churn but then the names are very distinct.

IIRC Domenico had that in an earlier internal version. That was still
confusing because both sets of members are used for polling, not just
the rt-thread bits. I suggested to rename them all to make it clear
which poll bits are for the rt thread and which ones are from the
aggregator thread.

IMO code clarity trumps churn avoidance here.
  

On Thu, Mar 23, 2023 at 09:55:11AM -0700, Suren Baghdasaryan wrote:
> On Thu, Mar 23, 2023 at 3:34 AM Domenico Cerasuolo
> > @@ -1254,16 +1262,19 @@ int psi_show(struct seq_file *m, struct psi_group *group, enum psi_res res)
> >  }
> >
> >  struct psi_trigger *psi_trigger_create(struct psi_group *group,
> > -                       char *buf, enum psi_res res)
> > +                       char *buf, enum psi_res res, struct file *file)
> >  {
> >         struct psi_trigger *t;
> >         enum psi_states state;
> >         u32 threshold_us;
> > +       bool privileged;
> >         u32 window_us;
> >
> >         if (static_branch_likely(&psi_disabled))
> >                 return ERR_PTR(-EOPNOTSUPP);
> >
> > +       privileged = cap_raised(file->f_cred->cap_effective, CAP_SYS_RESOURCE);
> 
> I missed one detail here. We are moving the cap check from open() to
> write(). That might break potential users which open the file from a
> process with that cap and then pass that FD to an unprivileged process
> to create the trigger by writing to that file. I'm not aware of any
> use of such a pattern but it is possible there are such users.
> With this change such users would have to delegate trigger creation to
> the privileged process too and the received FD would be used only for
> polling. IMHO that's a safer pattern because triggers are created by
> the privileged process.

Oh, it's checking file->f_cred, which is set up at open(). So if the
opener is privileged, the write can be delegated to an unprivileged
process.

But I agree that this is subtle and could use a comment.

This was a usecase specifically requested by Christian, actually.
  

On Thu, Mar 23, 2023 at 10:45 AM Johannes Weiner <hannes@cmpxchg.org> wrote:
>
> On Thu, Mar 23, 2023 at 09:55:11AM -0700, Suren Baghdasaryan wrote:
> > On Thu, Mar 23, 2023 at 3:34 AM Domenico Cerasuolo
> > > @@ -1254,16 +1262,19 @@ int psi_show(struct seq_file *m, struct psi_group *group, enum psi_res res)
> > >  }
> > >
> > >  struct psi_trigger *psi_trigger_create(struct psi_group *group,
> > > -                       char *buf, enum psi_res res)
> > > +                       char *buf, enum psi_res res, struct file *file)
> > >  {
> > >         struct psi_trigger *t;
> > >         enum psi_states state;
> > >         u32 threshold_us;
> > > +       bool privileged;
> > >         u32 window_us;
> > >
> > >         if (static_branch_likely(&psi_disabled))
> > >                 return ERR_PTR(-EOPNOTSUPP);
> > >
> > > +       privileged = cap_raised(file->f_cred->cap_effective, CAP_SYS_RESOURCE);
> >
> > I missed one detail here. We are moving the cap check from open() to
> > write(). That might break potential users which open the file from a
> > process with that cap and then pass that FD to an unprivileged process
> > to create the trigger by writing to that file. I'm not aware of any
> > use of such a pattern but it is possible there are such users.
> > With this change such users would have to delegate trigger creation to
> > the privileged process too and the received FD would be used only for
> > polling. IMHO that's a safer pattern because triggers are created by
> > the privileged process.
>
> Oh, it's checking file->f_cred, which is set up at open(). So if the
> opener is privileged, the write can be delegated to an unprivileged
> process.
>
> But I agree that this is subtle and could use a comment.
>
> This was a usecase specifically requested by Christian, actually.

Ah, great!