On Wed, Mar 19, 2025 at 11:47:32AM +0100, Mateusz Guzik wrote: > On Wed, Mar 19, 2025 at 12:13:30AM -0700, Greg Thelen wrote: > > From: Eric Dumazet <edumazet@xxxxxxxxxx> > > > > cgroup_rstat_flush_locked() grabs the irq safe cgroup_rstat_lock while > > iterating all possible cpus. It only drops the lock if there is > > scheduler or spin lock contention. If neither, then interrupts can be > > disabled for a long time. On large machines this can disable interrupts > > for a long enough time to drop network packets. On 400+ CPU machines > > I've seen interrupt disabled for over 40 msec. > > > > Prevent rstat from disabling interrupts while processing all possible > > cpus. Instead drop and reacquire cgroup_rstat_lock for each cpu. This > > approach was previously discussed in > > https://lore.kernel.org/lkml/ZBz%2FV5a7%2F6PZeM7S@xxxxxxxxxxxxxxx/, > > though this was in the context of an non-irq rstat spin lock. > > > > Benchmark this change with: > > 1) a single stat_reader process with 400 threads, each reading a test > > memcg's memory.stat repeatedly for 10 seconds. > > 2) 400 memory hog processes running in the test memcg and repeatedly > > charging memory until oom killed. Then they repeat charging and oom > > killing. > > > > v6.14-rc6 with CONFIG_IRQSOFF_TRACER with stat_reader and hogs, finds > > interrupts are disabled by rstat for 45341 usec: > > # => started at: _raw_spin_lock_irq > > # => ended at: cgroup_rstat_flush > > # > > # > > # _------=> CPU# > > # / _-----=> irqs-off/BH-disabled > > # | / _----=> need-resched > > # || / _---=> hardirq/softirq > > # ||| / _--=> preempt-depth > > # |||| / _-=> migrate-disable > > # ||||| / delay > > # cmd pid |||||| time | caller > > # \ / |||||| \ | / > > stat_rea-96532 52d.... 0us*: _raw_spin_lock_irq > > stat_rea-96532 52d.... 45342us : cgroup_rstat_flush > > stat_rea-96532 52d.... 45342us : tracer_hardirqs_on <-cgroup_rstat_flush > > stat_rea-96532 52d.... 45343us : <stack trace> > > => memcg1_stat_format > > => memory_stat_format > > => memory_stat_show > > => seq_read_iter > > => vfs_read > > => ksys_read > > => do_syscall_64 > > => entry_SYSCALL_64_after_hwframe > > > > With this patch the CONFIG_IRQSOFF_TRACER doesn't find rstat to be the > > longest holder. The longest irqs-off holder has irqs disabled for > > 4142 usec, a huge reduction from previous 45341 usec rstat finding. > > > > Running stat_reader memory.stat reader for 10 seconds: > > - without memory hogs: 9.84M accesses => 12.7M accesses > > - with memory hogs: 9.46M accesses => 11.1M accesses > > The throughput of memory.stat access improves. > > > > The mode of memory.stat access latency after grouping by of 2 buckets: > > - without memory hogs: 64 usec => 16 usec > > - with memory hogs: 64 usec => 8 usec > > The memory.stat latency improves. > > > > Signed-off-by: Eric Dumazet <edumazet@xxxxxxxxxx> > > Signed-off-by: Greg Thelen <gthelen@xxxxxxxxxx> > > Tested-by: Greg Thelen <gthelen@xxxxxxxxxx> > > --- > > kernel/cgroup/rstat.c | 12 +++++------- > > 1 file changed, 5 insertions(+), 7 deletions(-) > > > > diff --git a/kernel/cgroup/rstat.c b/kernel/cgroup/rstat.c > > index aac91466279f..976c24b3671a 100644 > > --- a/kernel/cgroup/rstat.c > > +++ b/kernel/cgroup/rstat.c > > @@ -323,13 +323,11 @@ static void cgroup_rstat_flush_locked(struct cgroup *cgrp) > > rcu_read_unlock(); > > } > > > > - /* play nice and yield if necessary */ > > - if (need_resched() || spin_needbreak(&cgroup_rstat_lock)) { > > - __cgroup_rstat_unlock(cgrp, cpu); > > - if (!cond_resched()) > > - cpu_relax(); > > - __cgroup_rstat_lock(cgrp, cpu); > > - } > > + /* play nice and avoid disabling interrupts for a long time */ > > + __cgroup_rstat_unlock(cgrp, cpu); > > + if (!cond_resched()) > > + cpu_relax(); > > + __cgroup_rstat_lock(cgrp, cpu); > > } > > } > > Is not this going a little too far? > > the lock + irq trip is quite expensive in its own right and now is > going to be paid for each cpu, as in the total time spent executing > cgroup_rstat_flush_locked is going to go up. > > Would your problem go away toggling this every -- say -- 8 cpus? I was concerned about this too, and about more lock bouncing, but the testing suggests that this actually overall improves the latency of cgroup_rstat_flush_locked() (at least on tested HW). So I don't think we need to do something like this unless a regression is observed. > > Just a suggestion. >
