Messages in this thread | | | Date | Sun, 5 Jun 2022 22:58:15 -0400 | Subject | Re: [PATCH v6 3/3] blk-cgroup: Optimize blkcg_rstat_flush() | From | Waiman Long <> |
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On 6/5/22 22:23, Ming Lei wrote: > On Sun, Jun 05, 2022 at 09:59:50PM -0400, Waiman Long wrote: >> On 6/5/22 21:39, Ming Lei wrote: >>> On Sun, Jun 05, 2022 at 07:15:27PM -0400, Waiman Long wrote: >>>> On 6/3/22 23:58, Ming Lei wrote: >>>> >>>>> diff --git a/mm/memcontrol.c b/mm/memcontrol.c >>>>> index abec50f31fe6..8c4f204dbf5b 100644 >>>>> --- a/mm/memcontrol.c >>>>> +++ b/mm/memcontrol.c >>>>> @@ -622,7 +622,7 @@ static inline void memcg_rstat_updated(struct mem_cgroup *memcg, int val) >>>>> { >>>>> unsigned int x; >>>>> - cgroup_rstat_updated(memcg->css.cgroup, smp_processor_id()); >>>>> + cgroup_rstat_updated(memcg->css.cgroup, smp_processor_id(), NULL); >>>>> x = __this_cpu_add_return(stats_updates, abs(val)); >>>>> if (x > MEMCG_CHARGE_BATCH) { >>>> I think the rstat set of functions are doing that already. So flush will >>>> only call CPUs that have called cgroup_rstat_updated() before. However, one >>> Yeah, I guess the detail is in cgroup_rstat_cpu_pop_updated(), but the >>> percpu lock(raw_spin_lock_irqsave) is still required, and cgroup_rstat_cpu_pop_updated() >>> is still called even through there isn't any update on this CPU. >> Yes, I think we may need to add a bitmask of what controllers have updates >> in cgroup_rstat_cpu structure. >>>> deficiency that I am aware of is that there is no bitmap of which controller >>>> have update. The problem that I saw in cgroup v2 is that in a cgroup with >>>> both memory controller and block controller enabled, a >>>> cgroup_rstat_updated() call from memory cgroup later causes the rstat >>>> function to call into block cgroup flush method even though there is no >>>> update in the block controller. This is an area that needs improvement. >>>> >>>> Your code does allow the block controller to be aware of that and avoid >>>> further action, but I think it has to be done in the rstat code to be >>>> applicable to all controllers instead of just specific to block controller. >>> I guess it can be done by adding one percpu variable to 'struct cgroup'. >>> >>>> There is another problem that this approach. Suppose the system have 20 >>>> block devices and one of them has an IO operation. Now the flush method >>>> still needs to iterate all the 20 blkg's to do an update. The block >>>> controller is kind of special that the number of per-cgroup IO stats depends >>>> on the number of block devices present. Other controllers just have one set >>>> of stats per cgroup. >>> Yeah, and this one is really blkio specific issue, and your patch does >>> cover this one. Maybe you can add one callback to >>> cgroup_rstat_updated(), so the "blkg_iostat_set" instance is added into >>> percpu list under percpu lock of cgroup_rstat_cpu_lock, then the lockless >>> list isn't needed. >> The rstat API is generic. It may not be a good idea to put controller >> specific information into it. Yes, cgroup_rstat_cpu_lock is taken at the >> read side (flush). It may not taken on the write side (update). So it may > Both cgroup_rstat_flush_locked()/cgroup_rstat_updated() take the percpu > cgroup_rstat_cpu_lock, so the new invented lockless list can be > replaced with plain list.
cgroup_rstat_updated() should only take the percpu cgroup_rstat_cpu_lock the first time it transition from "!updated" to "updated". After that, it returns immediately without the the lock. With a regular list, you will have to take the lock every time a new block device has an update. So there isn't much saving on the update side. In general, the lock/unlock sequence has a bit more overhead than the lockless insertion. On the flush side, there may be a bit of saving, but it is not the fast path.
Cheers, Longman
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