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Date: Thu, 19 Nov 2015 17:21:06 +0100 From: Peter Zijlstra <peterz@...radead.org> To: Thomas Gleixner <tglx@...utronix.de> Cc: Ingo Molnar <mingo@...nel.org>, Frederic Weisbecker <fweisbec@...il.com>, linux-kernel@...r.kernel.org Subject: [PATCH] nohz: Clarify magic in tick_nohz_stop_sched_tick() While going through the nohz code I got stumped by some of it. This patch adds a few comments clarifying the code; based on discussion with Thomas. Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org> --- diff --git a/kernel/time/tick-sched.c b/kernel/time/tick-sched.c index 7c7ec4515983..cba9f3479fab 100644 --- a/kernel/time/tick-sched.c +++ b/kernel/time/tick-sched.c @@ -603,15 +603,31 @@ static ktime_t tick_nohz_stop_sched_tick(struct tick_sched *ts, /* * If the tick is due in the next period, keep it ticking or - * restart it proper. + * force prod the timer. */ delta = next_tick - basemono; if (delta <= (u64)TICK_NSEC) { tick.tv64 = 0; + /* + * We've not stopped the tick yet, and there's a timer in the + * next period, so no point in stopping it either, bail. + */ if (!ts->tick_stopped) goto out; + + /* + * If, OTOH, we did stop it, but there's a pending (expired) + * timer reprogram the timer hardware to fire now. + * + * We will not restart the tick proper, just prod the timer + * hardware into firing an interrupt to process the pending + * timers. Just like tick_irq_exit() will not restart the tick + * for 'normal' interrupts. + * + * Only once we exit the idle loop will we re-enable the tick, + * see tick_nohz_idle_exit(). + */ if (delta == 0) { - /* Tick is stopped, but required now. Enforce it */ tick_nohz_restart(ts, now); goto out; } -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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