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From:   Daniel Colascione <dancol@google.com>
Date:   Wed, 2 Oct 2019 19:08:16 -0700
Message-ID: <CAKOZuesKY_=qkSXfmDO_1ALaqQtU0kz5Z+fBh05c8BR7oCDxKw@mail.gmail.com>
Subject: Re: [PATCH] Make SPLIT_RSS_COUNTING configurable
To:     Qian Cai <cai@lca.pw>
Cc:     Tim Murray <timmurray@google.com>,
        Suren Baghdasaryan <surenb@google.com>,
        linux-mm@vger.kernel.org,
        linux-kernel <linux-kernel@vger.kernel.org>,
        linux-mm <linux-mm@kvack.org>
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On Wed, Oct 2, 2019 at 6:56 PM Qian Cai <cai@lca.pw> wrote:
> > On Oct 2, 2019, at 4:29 PM, Daniel Colascione <dancol@google.com> wrote:
> >
> > Adding the correct linux-mm address.
> >
> >
> >> On Wed, Oct 2, 2019 at 1:25 PM Daniel Colascione <dancol@google.com> wrote:
> >>
> >> Using the new config option, users can disable SPLIT_RSS_COUNTING to
> >> get increased accuracy in user-visible mm counters.
> >>
> >> Signed-off-by: Daniel Colascione <dancol@google.com>
> >> ---
> >> include/linux/mm.h            |  4 ++--
> >> include/linux/mm_types_task.h |  5 ++---
> >> include/linux/sched.h         |  2 +-
> >> kernel/fork.c                 |  2 +-
> >> mm/Kconfig                    | 11 +++++++++++
> >> mm/memory.c                   |  6 +++---
> >> 6 files changed, 20 insertions(+), 10 deletions(-)
> >>
> >> diff --git a/include/linux/mm.h b/include/linux/mm.h
> >> index cc292273e6ba..221395de3cb4 100644
> >> --- a/include/linux/mm.h
> >> +++ b/include/linux/mm.h
> >> @@ -1637,7 +1637,7 @@ static inline unsigned long get_mm_counter(struct mm_struct *mm, int member)
> >> {
> >>        long val = atomic_long_read(&mm->rss_stat.count[member]);
> >>
> >> -#ifdef SPLIT_RSS_COUNTING
> >> +#ifdef CONFIG_SPLIT_RSS_COUNTING
> >>        /*
> >>         * counter is updated in asynchronous manner and may go to minus.
> >>         * But it's never be expected number for users.
> >> @@ -1723,7 +1723,7 @@ static inline void setmax_mm_hiwater_rss(unsigned long *maxrss,
> >>                *maxrss = hiwater_rss;
> >> }
> >>
> >> -#if defined(SPLIT_RSS_COUNTING)
> >> +#ifdef CONFIG_SPLIT_RSS_COUNTING
> >> void sync_mm_rss(struct mm_struct *mm);
> >> #else
> >> static inline void sync_mm_rss(struct mm_struct *mm)
> >> diff --git a/include/linux/mm_types_task.h b/include/linux/mm_types_task.h
> >> index c1bc6731125c..d2adc8057e65 100644
> >> --- a/include/linux/mm_types_task.h
> >> +++ b/include/linux/mm_types_task.h
> >> @@ -48,14 +48,13 @@ enum {
> >>        NR_MM_COUNTERS
> >> };
> >>
> >> -#if USE_SPLIT_PTE_PTLOCKS && defined(CONFIG_MMU)
> >> -#define SPLIT_RSS_COUNTING
> >> +#ifdef CONFIG_SPLIT_RSS_COUNTING
> >> /* per-thread cached information, */
> >> struct task_rss_stat {
> >>        int events;     /* for synchronization threshold */
> >>        int count[NR_MM_COUNTERS];
> >> };
> >> -#endif /* USE_SPLIT_PTE_PTLOCKS */
> >> +#endif /* CONFIG_SPLIT_RSS_COUNTING */
> >>
> >> struct mm_rss_stat {
> >>        atomic_long_t count[NR_MM_COUNTERS];
> >> diff --git a/include/linux/sched.h b/include/linux/sched.h
> >> index 2c2e56bd8913..22f354774540 100644
> >> --- a/include/linux/sched.h
> >> +++ b/include/linux/sched.h
> >> @@ -729,7 +729,7 @@ struct task_struct {
> >>        /* Per-thread vma caching: */
> >>        struct vmacache                 vmacache;
> >>
> >> -#ifdef SPLIT_RSS_COUNTING
> >> +#ifdef CONFIG_SPLIT_RSS_COUNTING
> >>        struct task_rss_stat            rss_stat;
> >> #endif
> >>        int                             exit_state;
> >> diff --git a/kernel/fork.c b/kernel/fork.c
> >> index f9572f416126..fc5e0889922b 100644
> >> --- a/kernel/fork.c
> >> +++ b/kernel/fork.c
> >> @@ -1917,7 +1917,7 @@ static __latent_entropy struct task_struct *copy_process(
> >>        p->vtime.state = VTIME_INACTIVE;
> >> #endif
> >>
> >> -#if defined(SPLIT_RSS_COUNTING)
> >> +#ifdef CONFIG_SPLIT_RSS_COUNTING
> >>        memset(&p->rss_stat, 0, sizeof(p->rss_stat));
> >> #endif
> >>
> >> diff --git a/mm/Kconfig b/mm/Kconfig
> >> index a5dae9a7eb51..372ef9449924 100644
> >> --- a/mm/Kconfig
> >> +++ b/mm/Kconfig
> >> @@ -736,4 +736,15 @@ config ARCH_HAS_PTE_SPECIAL
> >> config ARCH_HAS_HUGEPD
> >>        bool
> >>
> >> +config SPLIT_RSS_COUNTING
> >> +       bool "Per-thread mm counter caching"
> >> +       depends on MMU
> >> +       default y if NR_CPUS >= SPLIT_PTLOCK_CPUS
> >> +       help
> >> +         Cache mm counter updates in thread structures and
> >> +         flush them to visible per-process statistics in batches.
> >> +         Say Y here to slightly reduce cache contention in processes
> >> +         with many threads at the expense of decreasing the accuracy
> >> +         of memory statistics in /proc.
> >> +
> >> endmenu
>
> All those vague words are going to make developers almost impossible to decide the right selection here. It sounds like we should kill SPLIT_RSS_COUNTING at all to simplify the code as the benefit is so small vs the side-effect?

Killing SPLIT_RSS_COUNTING would be my first choice; IME, on mobile
and a basic desktop, it doesn't make a difference. I figured making it
a knob would help allay concerns about the performance impact in more
extreme configurations.