linux-kernel - Re: [PATCH v2] timers: Use only bucket expiry for base->next

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Date:   Wed, 15 Jul 2020 14:40:55 +0200
From:   Frederic Weisbecker <frederic@...nel.org>
To:     Anna-Maria Behnsen <anna-maria@...utronix.de>
Cc:     linux-kernel@...r.kernel.org,
        Peter Zijlstra <peterz@...radead.org>,
        Juri Lelli <juri.lelli@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH v2] timers: Use only bucket expiry for base->next_expiry
 value

On Tue, Jul 14, 2020 at 09:29:24AM +0200, Anna-Maria Behnsen wrote:
> The bucket expiry time is the effective expriy time of timers and is
> greater than or equal to the requested timer expiry time. This is due
> to the guarantee that timers never expire early and the reduced expiry
> granularity in the secondary wheel levels.
> 
> When a timer is enqueued, trigger_dyntick_cpu() checks whether the
> timer is the new first timer. This check compares next_expiry with
> the requested timer expiry value and not with the effective expiry
> value of the bucket into which the timer was queued.
> 
> Storing the requested timer expiry value in base->next_expiry can lead
> to base->clk going backwards if the requested timer expiry value is
> smaller than base->clk. Commit 30c66fc30ee7 ("timer: Prevent base->clk
> from moving backward") worked around this by preventing the store when
> timer->expiry is before base->clk, but did not fix the underlying
> problem.
> 
> Use the expiry value of the bucket into which the timer is queued to
> do the new first timer check. This fixes the base->clk going backward
> problem.
> 
> The workaround of commit 30c66fc30ee7 ("timer: Prevent base->clk from
> moving backward") in trigger_dyntick_cpu() is not longer necessary as the
> timers bucket expiry is guaranteed to be greater than or equal base->clk.
> 
> Signed-off-by: Anna-Maria Behnsen <anna-maria@...utronix.de>

Reviewed-by: Frederic Weisbecker <frederic@...nel.org>

Thanks a lot!