Messages in this thread | | | Date | Thu, 27 Apr 2023 15:09:58 +0100 | Subject | Re: [PATCH v3 08/19] x86/resctrl: Add cpumask_any_housekeeping() for limbo/overflow | From | James Morse <> |
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Hi Ilpo,
On 21/03/2023 15:14, Ilpo Järvinen wrote: > On Mon, 20 Mar 2023, James Morse wrote: > >> The limbo and overflow code picks a CPU to use from the domain's list >> of online CPUs. Work is then scheduled on these CPUs to maintain >> the limbo list and any counters that may overflow. >> >> cpumask_any() may pick a CPU that is marked nohz_full, which will >> either penalise the work that CPU was dedicated to, or delay the >> processing of limbo list or counters that may overflow. Perhaps >> indefinitely. Delaying the overflow handling will skew the bandwidth >> values calculated by mba_sc, which expects to be called once a second. >> >> Add cpumask_any_housekeeping() as a replacement for cpumask_any() >> that prefers housekeeping CPUs. This helper will still return >> a nohz_full CPU if that is the only option. The CPU to use is >> re-evaluated each time the limbo/overflow work runs. This ensures >> the work will move off a nohz_full CPU once a houskeeping CPU is > > housekeeping > >> available.
>> diff --git a/arch/x86/kernel/cpu/resctrl/internal.h b/arch/x86/kernel/cpu/resctrl/internal.h >> index 87545e4beb70..0b5fd5a0cda2 100644 >> --- a/arch/x86/kernel/cpu/resctrl/internal.h >> +++ b/arch/x86/kernel/cpu/resctrl/internal.h
>> +/** >> + * cpumask_any_housekeeping() - Chose any cpu in @mask, preferring those that >> + * aren't marked nohz_full >> + * @mask: The mask to pick a CPU from. >> + * >> + * Returns a CPU in @mask. If there are houskeeping CPUs that don't use >> + * nohz_full, these are preferred. >> + */ >> +static inline unsigned int cpumask_any_housekeeping(const struct cpumask *mask) >> +{ >> + int cpu, hk_cpu; >> + >> + cpu = cpumask_any(mask); >> + if (tick_nohz_full_cpu(cpu)) { >> + hk_cpu = cpumask_nth_andnot(0, mask, tick_nohz_full_mask); > > Why cpumask_nth_and() is not enough here? ..._andnot() seems to alter > tick_nohz_full_mask which doesn't seem desirable?
tick_nohz_full_mask is the list of CPUs we should avoid. This wants to find the first cpu set in the domain mask, and clear in tick_nohz_full_mask.
Where does cpumask_nth_andnot() modify its arguments? Its arguments are const.
Thanks,
James
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