Re: [PATCH] cgroup/rstat: avoid disabling irqs for O(num_cpu)

[Yosry Ahmed <yosry.ahmed@linux.dev>]

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@google.com>
> > 
> > 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@slm.duckdns.org/,
> > 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@google.com>
> > Signed-off-by: Greg Thelen <gthelen@google.com>
> > Tested-by: Greg Thelen <gthelen@google.com>
> > ---
> >  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.
>