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From:   Marco Elver <elver@google.com>
Date:   Wed, 22 Nov 2023 21:35:44 +0100
Message-ID: <CANpmjNPvDhyEcc0DdxrL8hVd0rZ-J4k95R5M5AwoeSotg-HCVg@mail.gmail.com>
Subject: Re: [PATCH] kfence: Replace local_clock() with ktime_get_boot_fast_ns()
To:     Juntong Deng <juntong.deng@outlook.com>
Cc:     glider@google.com, dvyukov@google.com, akpm@linux-foundation.org,
        kasan-dev@googlegroups.com, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org,
        linux-kernel-mentees@lists.linuxfoundation.org
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On Wed, 22 Nov 2023 at 21:01, Juntong Deng <juntong.deng@outlook.com> wrote:
>
> The time obtained by local_clock() is the local CPU time, which may
> drift between CPUs and is not suitable for comparison across CPUs.
>
> It is possible for allocation and free to occur on different CPUs,
> and using local_clock() to record timestamps may cause confusion.

The same problem exists with printk logging.

> ktime_get_boot_fast_ns() is based on clock sources and can be used
> reliably and accurately for comparison across CPUs.

You may be right here, however, the choice of local_clock() was
deliberate: it's the same timestamp source that printk uses.

Also, on systems where there is drift, the arch selects
CONFIG_HAVE_UNSTABLE_SCHED_CLOCK (like on x86) and the drift is
generally bounded.

> Signed-off-by: Juntong Deng <juntong.deng@outlook.com>
> ---
>  mm/kfence/core.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/mm/kfence/core.c b/mm/kfence/core.c
> index 3872528d0963..041c03394193 100644
> --- a/mm/kfence/core.c
> +++ b/mm/kfence/core.c
> @@ -295,7 +295,7 @@ metadata_update_state(struct kfence_metadata *meta, enum kfence_object_state nex
>         track->num_stack_entries = num_stack_entries;
>         track->pid = task_pid_nr(current);
>         track->cpu = raw_smp_processor_id();
> -       track->ts_nsec = local_clock(); /* Same source as printk timestamps. */
> +       track->ts_nsec = ktime_get_boot_fast_ns();

You have ignored the comment placed here - now it's no longer the same
source as printk timestamps. I think not being able to correlate
information from KFENCE reports with timestamps in lines from printk
is worse.

For now, I have to Nack: Unless you can prove that
ktime_get_boot_fast_ns() can still be correlated with timestamps from
printk timestamps, I think this change only trades one problem for
another.

Thanks,
-- Marco