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

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Date:   Fri, 10 Jul 2020 23:44:59 +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] timers: Use only bucket expiry for base->next_expiry
 value

Hi Anna-Maria,

Nice change, it indeed makes more sense that way.
Just a few details below:

On Fri, Jul 10, 2020 at 05:46:22PM +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 and also prevents unnecessary softirq invocations when the
> timer->expiry is not equal to the bucket expiry time in case of a new
> first timer which is queued in a secondary wheel level.

I think there shouldn't be such unecessary softirq invocations. Either they
fire at the bucket expiry time or the timer expiry time, it doesn't make
much difference.

More important below:

> -static int calc_wheel_index(unsigned long expires, unsigned long clk)
> +static int calc_wheel_index(unsigned long expires, unsigned long clk,
> +			    unsigned long *bucket_expiry)
>  {
>  	unsigned long delta = expires - clk;
>  	unsigned int idx;
>  
>  	if (delta < LVL_START(1)) {
> -		idx = calc_index(expires, 0);
> +		idx = calc_index(expires, 0, bucket_expiry);
>  	} else if (delta < LVL_START(2)) {
> -		idx = calc_index(expires, 1);
> +		idx = calc_index(expires, 1, bucket_expiry);
>  	} else if (delta < LVL_START(3)) {
> -		idx = calc_index(expires, 2);
> +		idx = calc_index(expires, 2, bucket_expiry);
>  	} else if (delta < LVL_START(4)) {
> -		idx = calc_index(expires, 3);
> +		idx = calc_index(expires, 3, bucket_expiry);
>  	} else if (delta < LVL_START(5)) {
> -		idx = calc_index(expires, 4);
> +		idx = calc_index(expires, 4, bucket_expiry);
>  	} else if (delta < LVL_START(6)) {
> -		idx = calc_index(expires, 5);
> +		idx = calc_index(expires, 5, bucket_expiry);
>  	} else if (delta < LVL_START(7)) {
> -		idx = calc_index(expires, 6);
> +		idx = calc_index(expires, 6, bucket_expiry);
>  	} else if (LVL_DEPTH > 8 && delta < LVL_START(8)) {
> -		idx = calc_index(expires, 7);
> +		idx = calc_index(expires, 7, bucket_expiry);
>  	} else if ((long) delta < 0) {
>  		idx = clk & LVL_MASK;

You also need to handle that part. That's in fact the critical one  :)

I'll rebase my series on top of that.

Thanks!