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

[Greg Thelen <gthelen@google.com>]

On Wed, Mar 19, 2025 at 10:53 AM Shakeel Butt <shakeel.butt@linux.dev> 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.
>
> Which kernel was this observed on in production?
>
> >
> > Prevent rstat from disabling interrupts while processing all possible
> > cpus. Instead drop and reacquire cgroup_rstat_lock for each cpu.
>
> Doing for each cpu might be too extreme. Have you tried with some
> batching?
>
> > 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.
> >
>
> Though this benchmark seems too extreme but userspace holding off irqs
> for that long time is bad. BTW are these memory hoggers, creating anon
> memory or file memory? Is [z]swap enabled?

The memory hoggers were anon, without any form of swap.

I think the other questions were answered in other replies, but feel free to
re-ask and I'll provide details.


> For the long term, I think we can use make this work without disabling
> irqs, similar to how networking manages sock lock.
>
> > 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.
>
> So, things are improving even without batching. I wonder if there are
> less readers then how will this look like. Can you try with single
> reader as well?
>
> >
> > Signed-off-by: Eric Dumazet <edumazet@google.com>
> > Signed-off-by: Greg Thelen <gthelen@google.com>
> > Tested-by: Greg Thelen <gthelen@google.com>
>
> Reviewed-by: Shakeel Butt <shakeel.butt@linux.dev>
>