Messages in this thread | | | Date | Tue, 31 May 2022 15:22:02 -0400 | Subject | Re: [PATCH] blk-cgroup: Optimize blkcg_rstat_flush() | From | Waiman Long <> |
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On 5/31/22 15:03, Waiman Long wrote: > On 5/31/22 14:24, Tejun Heo wrote: >> Hello, Waiman. >> >> On Tue, May 31, 2022 at 02:18:21PM -0400, Waiman Long wrote: >>> For a system with many CPUs and block devices, the time to do >>> blkcg_rstat_flush() from cgroup_rstat_flush() can be rather long. It >>> can be especially problematic as interrupt is disabled during the >>> flush. >>> It was reported that it might take seconds in some extreme cases >>> leading >>> to hard lockup messages. >>> >>> As it is likely that not all the percpu blkg_iostat_set's has been >>> updated since the last flush, those stale blkg_iostat_set's don't need >>> to be flushed in this case. This patch optimizes blkcg_rstat_flush() >>> by checking the current sequence number against the one recorded since >>> the last flush and skip the blkg_iostat_set if the sequence number >>> hasn't changed. There is a slight chance that it may miss an update >>> that is being done in parallel, the new update will just have to wait >>> until the next flush. >>> >>> Signed-off-by: Waiman Long <longman@redhat.com> >>> --- >>> block/blk-cgroup.c | 18 +++++++++++++++--- >>> block/blk-cgroup.h | 1 + >>> 2 files changed, 16 insertions(+), 3 deletions(-) >>> >>> diff --git a/block/blk-cgroup.c b/block/blk-cgroup.c >>> index 40161a3f68d0..79b89af61ef2 100644 >>> --- a/block/blk-cgroup.c >>> +++ b/block/blk-cgroup.c >>> @@ -864,11 +864,23 @@ static void blkcg_rstat_flush(struct >>> cgroup_subsys_state *css, int cpu) >>> unsigned long flags; >>> unsigned int seq; >>> + seq = u64_stats_fetch_begin(&bisc->sync); >>> + /* >>> + * If the sequence number hasn't been updated since the last >>> + * flush, we can skip this blkg_iostat_set though we may miss >>> + * an update that is happening in parallel. >>> + */ >>> + if (seq == bisc->last_seq) >>> + continue; >> Is this a sufficient solution? The code assumes that there aren't too >> many >> blkgs for the cgroup, which can be wrong in some cases. Wouldn't it be >> better to create a list of updated blkg's per blkcg so that we don't >> walk >> all the dormant ones? > > It is probably not a sufficient solution, but it is simple. The > problem with keeping a list of recently updated blkg's is that > sequence lock does not provide enough synchronization on the read side > to guarantee a race free reset of the list. It may be doable, but I > need to think harder on the best way to do it without too much overhead. > > Thanks, > Longman > I think we can use a lockless list to keep a list of recently updated blkg for each blkcg. There is an atomic operation overhead when an entry is added to the list though. I will send an updated patch later today or tomorrow.
Thanks, Longman
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