Messages in this thread | | | Date | Tue, 31 May 2022 15:03:08 -0400 | Subject | Re: [PATCH] blk-cgroup: Optimize blkcg_rstat_flush() | From | Waiman Long <> |
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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
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