Messages in this thread | | | Date | Thu, 27 Apr 2023 15:09:52 +0100 | Subject | Re: [PATCH v3 08/19] x86/resctrl: Add cpumask_any_housekeeping() for limbo/overflow | From | James Morse <> |
| |
Hi Ilpo,
On 21/03/2023 13:21, 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 >> 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 >> @@ -55,6 +56,28 @@ >> /* Max event bits supported */ >> #define MAX_EVT_CONFIG_BITS GENMASK(6, 0) >> >> +/** >> + * 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); >> + if (hk_cpu < nr_cpu_ids) >> + cpu = hk_cpu; >> + } >> + >> + return cpu; >> +} >> diff --git a/include/linux/tick.h b/include/linux/tick.h >> index bfd571f18cfd..ae2e9019fc18 100644 >> --- a/include/linux/tick.h >> +++ b/include/linux/tick.h >> @@ -174,9 +174,10 @@ static inline u64 get_cpu_iowait_time_us(int cpu, u64 *unused) { return -1; } >> static inline void tick_nohz_idle_stop_tick_protected(void) { } >> #endif /* !CONFIG_NO_HZ_COMMON */ >> >> +extern cpumask_var_t tick_nohz_full_mask; >> + >> #ifdef CONFIG_NO_HZ_FULL >> extern bool tick_nohz_full_running; >> -extern cpumask_var_t tick_nohz_full_mask; > > Its definition seems to also be inside #ifdef: > > kernel/time/tick-sched.c-#ifdef CONFIG_NO_HZ_FULL > kernel/time/tick-sched.c:cpumask_var_t tick_nohz_full_mask; > kernel/time/tick-sched.c:EXPORT_SYMBOL_GPL(tick_nohz_full_mask);
Indeed, but all the uses are guarded by tick_nohz_full_cpu(), which the compiler knows is false if CONFIG_NO_HZ_FULL is not selected.
Moving the prototype is enough to let the compiler parse the code to check its correct, before dead-code-eliminating it. There is no need to carry around the cpumask if its never going to be used. This would only cause a problem if someone adds a user of tick_nohz_full_mask which isn't guarded by IS_ENABLED(). I argue that would be a bug.
All this is being done to avoid more #ifdeffery!)
Thanks,
James
| |