Re: [RFC PATCH v2 0/2] mm: convert mm's rss stats to use atomic mode

[Dennis Zhou]

Hi Peng,

On Wed, Apr 24, 2024 at 12:29:25PM +0800, zhangpeng (AS) wrote:
> On 2024/4/18 22:20, Peng Zhang wrote:
> 
> Any suggestions or opinions are welcome. Could someone please review
> this patch series?
> Thanks!
> 

Sorry, I haven't been very active lately. This is what I remember
discussing a while back. I'll take a close look tomorrow.

Thanks,
Dennis

> > From: ZhangPeng <zhangpeng362@xxxxxxxxxx>
> > 
> > Since commit f1a7941243c1 ("mm: convert mm's rss stats into
> > percpu_counter"), the rss_stats have converted into percpu_counter,
> > which convert the error margin from (nr_threads * 64) to approximately
> > (nr_cpus ^ 2). However, the new percpu allocation in mm_init() causes a
> > performance regression on fork/exec/shell. Even after commit 14ef95be6f55
> > ("kernel/fork: group allocation/free of per-cpu counters for mm struct"),
> > the performance of fork/exec/shell is still poor compared to previous
> > kernel versions.
> > 
> > To mitigate performance regression, we delay the allocation of percpu
> > memory for rss_stats. Therefore, we convert mm's rss stats to use
> > percpu_counter atomic mode. For single-thread processes, rss_stat is in
> > atomic mode, which reduces the memory consumption and performance
> > regression caused by using percpu. For multiple-thread processes,
> > rss_stat is switched to the percpu mode to reduce the error margin.
> > We convert rss_stats from atomic mode to percpu mode only when the
> > second thread is created.
> > 
> > After lmbench test, we can get 2% ~ 4% performance improvement
> > for lmbench fork_proc/exec_proc/shell_proc and 6.7% performance
> > improvement for lmbench page_fault (before batch mode[1]).
> > 
> > The test results are as follows:
> >               base           base+revert        base+this patch
> > 
> > fork_proc    416.3ms        400.0ms  (3.9%)    398.6ms  (4.2%)
> > exec_proc    2095.9ms       2061.1ms (1.7%)    2047.7ms (2.3%)
> > shell_proc   3028.2ms       2954.7ms (2.4%)    2961.2ms (2.2%)
> > page_fault   0.3603ms       0.3358ms (6.8%)    0.3361ms (6.7%)
> > 
> > [1] https://lore.kernel.org/all/20240412064751.119015-1-wangkefeng.wang@xxxxxxxxxx/
> > 
> > ChangeLog:
> > v2->v1:
> > - Convert rss_stats from atomic mode to percpu mode only when
> >    the second thread is created per Jan Kara.
> > - Compared with v1, the performance data may be different due to
> >    different test machines.
> > 
> > ZhangPeng (2):
> >    percpu_counter: introduce atomic mode for percpu_counter
> >    mm: convert mm's rss stats to use atomic mode
> > 
> >   include/linux/mm.h             | 50 +++++++++++++++++++++++++++++-----
> >   include/linux/percpu_counter.h | 43 +++++++++++++++++++++++++++--
> >   include/trace/events/kmem.h    |  4 +--
> >   kernel/fork.c                  | 18 +++++++-----
> >   lib/percpu_counter.c           | 31 +++++++++++++++++++--
> >   5 files changed, 125 insertions(+), 21 deletions(-)
> > 
> -- 
> Best Regards,
> Peng
>