linux-kernel - Re: [PATCH v2] sched/fair: Limit sched_cfs_period

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Date:   Thu, 21 Mar 2019 19:01:37 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Phil Auld <pauld@...hat.com>
Cc:     linux-kernel@...r.kernel.org, Ben Segall <bsegall@...gle.com>,
        Ingo Molnar <mingo@...hat.com>,
        Anton Blanchard <anton@...abs.org>
Subject: Re: [PATCH v2] sched/fair: Limit sched_cfs_period_timer loop to
 avoid hard lockup

On Tue, Mar 19, 2019 at 09:00:05AM -0400, Phil Auld wrote:
> sched/fair: Limit sched_cfs_period_timer loop to avoid hard lockup
> 
> With extremely short cfs_period_us setting on a parent task group with a large
> number of children the for loop in sched_cfs_period_timer can run until the
> watchdog fires. There is no guarantee that the call to hrtimer_forward_now()
> will ever return 0.  The large number of children can make
> do_sched_cfs_period_timer() take longer than the period.

> 
> To prevent this we add protection to the loop that detects when the loop has run
> too many times and scales the period and quota up, proportionally, so that the timer
> can complete before then next period expires.  This preserves the relative runtime
> quota while preventing the hard lockup.
> 
> A warning is issued reporting this state and the new values.
> 
> v2: Math reworked/simplified by Peter Zijlstra.
> 
> Signed-off-by: Phil Auld <pauld@...hat.com>
> Cc: Ben Segall <bsegall@...gle.com>
> Cc: Ingo Molnar <mingo@...hat.com>
> Cc: Peter Zijlstra (Intel) <peterz@...radead.org>
> Cc: Anton Blanchard <anton@...abs.org>

Thanks!