Re: [PATCH] mm/lruvec: trace LRU add drains and drain-all queuing

[Shakeel Butt <shakeel.butt@linux.dev>]

On Wed, Jun 10, 2026 at 12:20:19PM -0700, JP Kobryn wrote:
> On 6/10/26 11:54 AM, JP Kobryn wrote:
> > On 6/9/26 6:21 PM, Shakeel Butt wrote:
> >> On Tue, Jun 09, 2026 at 05:16:15PM -0700, JP Kobryn wrote:
> >>> On 6/9/26 5:07 PM, JP Kobryn wrote:
> >>>> On 6/9/26 12:44 AM, Barry Song wrote:
> >>>>> On Tue, Jun 9, 2026 at 12:12 PM JP Kobryn <jp.kobryn@linux.dev> wrote:
> >>>>>>

[...]

> >>>>> Do you need tracing on each CPU individually, or is tracing the
> >>>>> entire __lru_add_drain_all() invocation sufficient?
> >>>>
> >>>> I think the latter would be fine. The remote work will invoke the
> >>>> mm_lru_add_drain tracepoint, which will show up as kworker stacks. Since
> >>>> the event already has the CPU, we could see where queued drains actually
> >>>> ran.
> >>>
> >>> Actually if it's just a single invocation and the only event data is the
> >>> force flag, a tracepoint may not even be needed. Other probes can be
> >>> installed on function invocation and read the single argument. I can
> >>> drop this from v2 and keep the single mm_lru_add_drain tracepoint.
> >>
> >> No we do want to trace the callers requesting to drain from all the CPUs. If you
> >> trace just lru_add_drain_cpu() then you will only see that the drain is
> >> requested for a given CPU but no information on the requester. 
> >>
> >> Also as Barry said, I think single trace for whole __lru_add_drain_all() is good
> >> enough.
> > 
> > Right, but couldn't that already be done with fentry or kprobe? If we
> > only need the calling stack and the argument value of force_all_cpus I
> > don't see a strong need for a dedicated tracepoint.
> 
> Nevermind that. I see it's declared inline so I'll add a tracepoint and
> send v3.

Thanks. BTW even without inline keyword, compiler can still decide to inline
a function, so kprobe/fentry are not always reliable.
>