Messages in this thread Patch in this message | | | Date | Thu, 14 Dec 2023 11:37:52 +0000 | Subject | Re: [PATCH v7 14/24] x86/resctrl: Allow resctrl_arch_rmid_read() to sleep | From | James Morse <> |
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Hi Babu,
On 09/11/2023 20:42, Moger, Babu wrote: > On 10/25/23 13:03, James Morse wrote: >> MPAM's cache occupancy counters can take a little while to settle once >> the monitor has been configured. The maximum settling time is described >> to the driver via a firmware table. The value could be large enough >> that it makes sense to sleep. To avoid exposing this to resctrl, it >> should be hidden behind MPAM's resctrl_arch_rmid_read(). >> >> resctrl_arch_rmid_read() may be called via IPI meaning it is unable >> to sleep. In this case resctrl_arch_rmid_read() should return an error >> if it needs to sleep. This will only affect MPAM platforms where >> the cache occupancy counter isn't available immediately, nohz_full is >> in use, and there are no housekeeping CPUs in the necessary domain. >> >> There are three callers of resctrl_arch_rmid_read(): >> __mon_event_count() and __check_limbo() are both called from a >> non-migrateable context. mon_event_read() invokes __mon_event_count() >> using smp_call_on_cpu(), which adds work to the target CPUs workqueue. >> rdtgroup_mutex() is held, meaning this cannot race with the resctrl >> cpuhp callback. __check_limbo() is invoked via schedule_delayed_work_on() >> also adds work to a per-cpu workqueue. >> >> The remaining call is add_rmid_to_limbo() which is called in response >> to a user-space syscall that frees an RMID. This opportunistically >> reads the LLC occupancy counter on the current domain to see if the >> RMID is over the dirty threshold. This has to disable preemption to >> avoid reading the wrong domain's value. Disabling pre-emption here >> prevents resctrl_arch_rmid_read() from sleeping.
> I dont know what did you mean by "This has to disable preemption to > avoid reading the wrong domain's value."
Pre-emption lets this thread be scheduled out, and potentially scheduled back in on a different CPU, possibly in a different domain. Any code with the concept of 'this domain' has to to ensure it can't be migrated. Disabling pre-emption is the most common way of doing that.
Disabling pre-emption also prevents the thread from sleeping, because it can't be scheduled out.
> Who is disabling the preemption here? Is that specific to ARM? > Can you please make that clear? Or Am i missing something?
add_rmid_to_limbo() is calling get_cpu(), which raises the pre-empt counter. If it only wanted the CPU number it could have just called smp_processor_id() - but that wouldn't be safe because the thread can be migrated, meaning the cpu number can change.
All this is to ensure that cpumask_test_cpu() and resctrl_arch_rmid_read() run on the same CPU.
Thanks,
James
>> add_rmid_to_limbo() walks each domain, but only reads the counter >> on one domain. If the system has more than one domain, the RMID will >> always be added to the limbo list. If the RMIDs usage was not over the >> threshold, it will be removed from the list when __check_limbo() runs. >> Make this the default behaviour. Free RMIDs are always added to the >> limbo list for each domain. >> >> The user visible effect of this is that a clean RMID is not available >> for re-allocation immediately after 'rmdir()' completes, this behaviour >> was never portable as it never happened on a machine with multiple >> domains. >> >> Removing this path allows resctrl_arch_rmid_read() to sleep if its called >> with interrupts unmasked. Document this is the expected behaviour, and >> add a might_sleep() annotation to catch changes that won't work on arm64.
>> diff --git a/arch/x86/kernel/cpu/resctrl/monitor.c b/arch/x86/kernel/cpu/resctrl/monitor.c >> index fa3319021881..409817b0ae2c 100644 >> --- a/arch/x86/kernel/cpu/resctrl/monitor.c >> +++ b/arch/x86/kernel/cpu/resctrl/monitor.c >> @@ -464,17 +464,7 @@ static void add_rmid_to_limbo(struct rmid_entry *entry) >> idx = resctrl_arch_rmid_idx_encode(entry->closid, entry->rmid); >> >> entry->busy = 0; >> - cpu = get_cpu(); >> list_for_each_entry(d, &r->domains, list) { >> - if (cpumask_test_cpu(cpu, &d->cpu_mask)) { >> - err = resctrl_arch_rmid_read(r, d, entry->closid, >> - entry->rmid, >> - QOS_L3_OCCUP_EVENT_ID, >> - &val); >> - if (err || val <= resctrl_rmid_realloc_threshold) >> - continue; >> - } >> - >> /* >> * For the first limbo RMID in the domain, >> * setup up the limbo worker. >> @@ -484,15 +474,10 @@ static void add_rmid_to_limbo(struct rmid_entry *entry) >> set_bit(idx, d->rmid_busy_llc); >> entry->busy++; >> } >> - put_cpu(); >> >> - if (entry->busy) { >> - rmid_limbo_count++; >> - if (IS_ENABLED(CONFIG_RESCTRL_RMID_DEPENDS_ON_CLOSID)) >> - closid_num_dirty_rmid[entry->closid]++; >> - } else { >> - list_add_tail(&entry->list, &rmid_free_lru); >> - } >> + rmid_limbo_count++; >> + if (IS_ENABLED(CONFIG_RESCTRL_RMID_DEPENDS_ON_CLOSID)) >> + closid_num_dirty_rmid[entry->closid]++; >> } >>
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