On Wed, Mar 22, 2023 at 11:33 PM Shakeel Butt <shakeelb@xxxxxxxxxx> wrote: > > 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. I think it also protects the cpu base stats flushed by cgroup_base_stat_flush(). I will add a comment in the next version. > > > > > > 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. Right, but I think this would be orthogonal to this patch series. > > > > > 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. I wanted to start with VM_BUG_ON(!in_task()) but I wasn't sure that all contexts that call rstat flushing are not in irq contexts. I added should_skip_flush() so that if there are existing flushers in irq context, or new flushers are added, we are protected against a deadlock. We can change should_skip_flush() to have a WARN_ON_ONCE(!in_task()) to alert in this case. If you prefer removing should_skip_flush() and just adding VM_BUG_ON(!in_task()) we can do that, but personally I was not confident enough that we have no code paths today that may attempt flushing from irq context.
