On Wed, Mar 22, 2023 at 10:15 PM Yosry Ahmed <yosryahmed@xxxxxxxxxx> wrote: > [...] > > Couple of questions: > > > > 1. What exactly is cgroup_rstat_lock protecting? Can we just remove it > > altogether? > > I believe it protects the global state variables that we flush into. > For example, for memcg, it protects mem_cgroup->vmstats. > > I tried removing the lock and allowing concurrent flushing on > different cpus, by changing mem_cgroup->vmstats to use atomics > instead, but that turned out to be a little expensive. Also, > cgroup_rstat_lock is already contended by different flushers > (mitigated by stats_flush_lock on the memcg side). If we remove it, > concurrent flushers contend on every single percpu lock instead, which > also seems to be expensive. We should add a comment on what it is protecting. I think block rstat are fine but memcg and bpf would need this. > > > 2. Are we really calling rstat flush in irq context? > > I think it is possible through the charge/uncharge path: > memcg_check_events()->mem_cgroup_threshold()->mem_cgroup_usage(). I > added the protection against flushing in an interrupt context for > future callers as well, as it may cause a deadlock if we don't disable > interrupts when acquiring cgroup_rstat_lock. > > > 3. The mem_cgroup_flush_stats() call in mem_cgroup_usage() is only > > done for root memcg. Why is mem_cgroup_threshold() interested in root > > memcg usage? Why not ignore root memcg in mem_cgroup_threshold() ? > > I am not sure, but the code looks like event notifications may be set > up on root memcg, which is why we need to check thresholds. This is something we should deprecate as root memcg's usage is ill defined. > > Even if mem_cgroup_threshold() does not flush memcg stats, the purpose > of this patch is to make sure the rstat flushing code itself is not > disabling interrupts; which it currently does for any unsleepable > context, even if it is interruptible. Basically I am saying we should aim for VM_BUG_ON(!in_task()) in the flush function rather than adding should_skip_flush() which does not stop potential new irq flushers.
