Re: [PATCH] Revert "xen/cpufreq: fix usages of align_timer() in the on-demand governor"

[Jan Beulich <jbeulich@xxxxxxxx>]

On 15.06.2026 21:39, Jason Andryuk wrote:
> The original commit showed a ~6% regression in a benchmark.  The call to
> align_timer(firsttick, period) rounds firsttick up to the next mutiple
> of the period, if firsttick % period != 0:
> 
> align_timer(0, period)          -> 0
> align_timer(1, period)          -> period
> align_timer(period, period)     -> period
> align_timer(period + 1, period) -> 2 * period
> 
> So adding the period (sampling_rate) before calling align_timer() will
> in most cases incease the expiration to 2 * period (sampling_rate) (the
> exception being firsttick % period == 0).  This longer timer slows the
> reaction time of the algorithm.
> 
> This reverts commit a0ed5bcfbeee81c91c574ad484faa057054eaf09.
> 
> Signed-off-by: Jason Andryuk <jason.andryuk@xxxxxxx>
> ---
> This is backported in stable trees and should be reverted there as well
> (found in 4.20.3).
> 
> A Fixes seems superfluous and not normally used with a revert, but if
> needed:
> Fixes: a0ed5bcfbeee ("xen/cpufreq: fix usages of align_timer() in the 
> on-demand governor")

If "git grep" is intended to (also) find such straight reverts by merely
matching "Fixes: ", I think the tag should still be put there.

Talking of Fixes: tags - the original change had two of them, so there
must have been a problem? Or, Roger, was this purely an observation from
looking the the code?

> --- a/xen/drivers/cpufreq/cpufreq_ondemand.c
> +++ b/xen/drivers/cpufreq/cpufreq_ondemand.c
> @@ -185,8 +185,7 @@ static void cf_check do_dbs_timer(void *dbs)
>      dbs_check_cpu(dbs_info);
>  
>      set_timer(&per_cpu(dbs_timer, dbs_info->cpu),
> -              align_timer(NOW() + dbs_tuners_ins.sampling_rate,
> -                          dbs_tuners_ins.sampling_rate));
> +            align_timer(NOW() , dbs_tuners_ins.sampling_rate));
>  }

As much as I understand you wanting to have things simple by doing a
straight revert, imo the formatting flaws better wouldn't be introduced
again. If I ended up committing this, I'd very likely take the liberty
of doing so. Yet before ack-ing the question above needs answering.

Jan