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Wysocki" , Daniel Lezcano , Thomas Gleixner , Anna-Maria Behnsen Subject: [PATCH 05/10] cpuidle: Comment about timers requirements VS idle handler Date: Fri, 11 Aug 2023 19:00:44 +0200 Message-Id: <20230811170049.308866-6-frederic@kernel.org> X-Mailer: git-send-email 2.34.1 In-Reply-To: <20230811170049.308866-1-frederic@kernel.org> References: <20230811170049.308866-1-frederic@kernel.org> MIME-Version: 1.0 X-Spam-Status: No, score=-2.1 required=5.0 tests=BAYES_00,DKIMWL_WL_HIGH, DKIM_SIGNED,DKIM_VALID,DKIM_VALID_AU,DKIM_VALID_EF, RCVD_IN_DNSWL_BLOCKED,SPF_HELO_NONE,SPF_PASS autolearn=ham autolearn_force=no version=3.4.6 X-Spam-Checker-Version: SpamAssassin 3.4.6 (2021-04-09) on lindbergh.monkeyblade.net Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org X-getmail-retrieved-from-mailbox: INBOX X-GMAIL-THRID: 1773953194775668951 X-GMAIL-MSGID: 1773953194775668951 Signed-off-by: Frederic Weisbecker Acked-by: Rafael J. Wysocki --- kernel/sched/idle.c | 30 ++++++++++++++++++++++++++++++ 1 file changed, 30 insertions(+) diff --git a/kernel/sched/idle.c b/kernel/sched/idle.c index 342f58a329f5..d52f6e3e3854 100644 --- a/kernel/sched/idle.c +++ b/kernel/sched/idle.c @@ -258,6 +258,36 @@ static void do_idle(void) while (!need_resched()) { rmb(); + /* + * Interrupts shouldn't be re-enabled from that point on until + * the CPU sleeping instruction is reached. Otherwise an interrupt + * may fire and queue a timer that would be ignored until the CPU + * wakes from the sleeping instruction. And testing need_resched() + * doesn't tell about pending needed timer reprogram. + * + * Several cases to consider: + * + * - SLEEP-UNTIL-PENDING-INTERRUPT based instructions such as + * "wfi" or "mwait" are fine because they can be entered with + * interrupt disabled. + * + * - sti;mwait() couple is fine because the interrupts are + * re-enabled only upon the execution of mwait, leaving no gap + * in-between. + * + * - ROLLBACK based idle handlers with the sleeping instruction + * called with interrupts enabled are NOT fine. In this scheme + * when the interrupt detects it has interrupted an idle handler, + * it rolls back to its beginning which performs the + * need_resched() check before re-executing the sleeping + * instruction. This can leak a pending needed timer reprogram. + * If such a scheme is really mandatory due to the lack of an + * appropriate CPU sleeping instruction, then a FAST-FORWARD + * must instead be applied: when the interrupt detects it has + * interrupted an idle handler, it must resume to the end of + * this idle handler so that the generic idle loop is iterated + * again to reprogram the tick. + */ local_irq_disable(); if (cpu_is_offline(cpu)) {