Re: [PATCH v2] tick/nohz: Avoid unused timekeeping_max_deferment() calls

[Rik van Riel]

On Fri, 2026-09-18 at 06:34 -0700, Usama Arif wrote:
> tick_nohz_next_event() limits a CPU's sleep interval to the maximum
> deferment supported by the current clocksource when that CPU owns the
> do_timer() duty. If the duty is unassigned, the limit also applies
> when
> the CPU's TS_FLAG_DO_TIMER_LAST flag is set.
> 
> After the early timer checks, the function currently reads the
> maximum
> deferment unconditionally. It then replaces the result with KTIME_MAX
> unless one of the two conditions above applies.
> 
> timekeeping_max_deferment() performs a seqcount-protected read of the
> shared timekeeper and follows its clocksource pointer. Check the
> do_timer
> state first and avoid this work when the result would be discarded.
> This
> leaves the resulting expiry unchanged and reduces accesses to
> timekeeper
> data that is modified regularly.
> 
> On x86-64 this removes 18-20 dynamically executed instructions,
> including
> the call, from the common non-owner path when the seqcount does not
> retry.
> 
> Signed-off-by: Usama Arif <usama.arif@xxxxxxxxx>

The changelog confused me a little at first, but after
reading through the code it all made sense.

Reviewed-by: Rik van Riel <riel@xxxxxxxxxxx>

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