linux-kernel - Re: [PATCH v3] uprobes: turn trace_uprobe's nhit counter to be per-CPU one

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Message-Id: <20240905000243.c8549b30be33a3ad73eabf05@kernel.org>
Date: Thu, 5 Sep 2024 00:02:43 +0900
From: Masami Hiramatsu (Google) <mhiramat@...nel.org>
To: Andrii Nakryiko <andrii.nakryiko@...il.com>
Cc: Andrii Nakryiko <andrii@...nel.org>, linux-trace-kernel@...r.kernel.org,
 rostedt@...dmis.org, peterz@...radead.org, oleg@...hat.com,
 bpf@...r.kernel.org, linux-kernel@...r.kernel.org, jolsa@...nel.org
Subject: Re: [PATCH v3] uprobes: turn trace_uprobe's nhit counter to be
 per-CPU one

On Thu, 29 Aug 2024 10:28:24 -0700
Andrii Nakryiko <andrii.nakryiko@...il.com> wrote:

> On Tue, Aug 27, 2024 at 8:55 PM Masami Hiramatsu <mhiramat@...nel.org> wrote:
> >
> > On Tue, 13 Aug 2024 13:34:09 -0700
> > Andrii Nakryiko <andrii@...nel.org> wrote:
> >
> > > trace_uprobe->nhit counter is not incremented atomically, so its value
> > > is questionable in when uprobe is hit on multiple CPUs simultaneously.
> > >
> > > Also, doing this shared counter increment across many CPUs causes heavy
> > > cache line bouncing, limiting uprobe/uretprobe performance scaling with
> > > number of CPUs.
> > >
> > > Solve both problems by making this a per-CPU counter.
> > >
> >
> > Looks good to me. Let me pick it to linux-trace probes/for-next.
> >
> 
> Thanks! I just checked linux-trace repo, doesn't seem like this was
> applied yet, is that right? Or am I checking in the wrong place?

Sorry, I missed to push probes/for-next. Let me push it.

Thank you,

> 
> > Thank you,
> >
> >
> > > Reviewed-by: Oleg Nesterov <oleg@...hat.com>
> > > Signed-off-by: Andrii Nakryiko <andrii@...nel.org>
> > > ---
> > >  kernel/trace/trace_uprobe.c | 24 +++++++++++++++++++++---
> > >  1 file changed, 21 insertions(+), 3 deletions(-)
> > >
> > > diff --git a/kernel/trace/trace_uprobe.c b/kernel/trace/trace_uprobe.c
> > > index c98e3b3386ba..c3df411a2684 100644
> > > --- a/kernel/trace/trace_uprobe.c
> > > +++ b/kernel/trace/trace_uprobe.c
> > > @@ -17,6 +17,7 @@
> > >  #include <linux/string.h>
> > >  #include <linux/rculist.h>
> > >  #include <linux/filter.h>
> > > +#include <linux/percpu.h>
> > >
> > >  #include "trace_dynevent.h"
> > >  #include "trace_probe.h"
> > > @@ -62,7 +63,7 @@ struct trace_uprobe {
> > >       char                            *filename;
> > >       unsigned long                   offset;
> > >       unsigned long                   ref_ctr_offset;
> > > -     unsigned long                   nhit;
> > > +     unsigned long __percpu          *nhits;
> > >       struct trace_probe              tp;
> > >  };
> > >
> > > @@ -337,6 +338,12 @@ alloc_trace_uprobe(const char *group, const char *event, int nargs, bool is_ret)
> > >       if (!tu)
> > >               return ERR_PTR(-ENOMEM);
> > >
> > > +     tu->nhits = alloc_percpu(unsigned long);
> > > +     if (!tu->nhits) {
> > > +             ret = -ENOMEM;
> > > +             goto error;
> > > +     }
> > > +
> > >       ret = trace_probe_init(&tu->tp, event, group, true, nargs);
> > >       if (ret < 0)
> > >               goto error;
> > > @@ -349,6 +356,7 @@ alloc_trace_uprobe(const char *group, const char *event, int nargs, bool is_ret)
> > >       return tu;
> > >
> > >  error:
> > > +     free_percpu(tu->nhits);
> > >       kfree(tu);
> > >
> > >       return ERR_PTR(ret);
> > > @@ -362,6 +370,7 @@ static void free_trace_uprobe(struct trace_uprobe *tu)
> > >       path_put(&tu->path);
> > >       trace_probe_cleanup(&tu->tp);
> > >       kfree(tu->filename);
> > > +     free_percpu(tu->nhits);
> > >       kfree(tu);
> > >  }
> > >
> > > @@ -815,13 +824,21 @@ static int probes_profile_seq_show(struct seq_file *m, void *v)
> > >  {
> > >       struct dyn_event *ev = v;
> > >       struct trace_uprobe *tu;
> > > +     unsigned long nhits;
> > > +     int cpu;
> > >
> > >       if (!is_trace_uprobe(ev))
> > >               return 0;
> > >
> > >       tu = to_trace_uprobe(ev);
> > > +
> > > +     nhits = 0;
> > > +     for_each_possible_cpu(cpu) {
> > > +             nhits += per_cpu(*tu->nhits, cpu);
> > > +     }
> > > +
> > >       seq_printf(m, "  %s %-44s %15lu\n", tu->filename,
> > > -                     trace_probe_name(&tu->tp), tu->nhit);
> > > +                trace_probe_name(&tu->tp), nhits);
> > >       return 0;
> > >  }
> > >
> > > @@ -1512,7 +1529,8 @@ static int uprobe_dispatcher(struct uprobe_consumer *con, struct pt_regs *regs)
> > >       int ret = 0;
> > >
> > >       tu = container_of(con, struct trace_uprobe, consumer);
> > > -     tu->nhit++;
> > > +
> > > +     this_cpu_inc(*tu->nhits);
> > >
> > >       udd.tu = tu;
> > >       udd.bp_addr = instruction_pointer(regs);
> > > --
> > > 2.43.5
> > >
> >
> >
> > --
> > Masami Hiramatsu (Google) <mhiramat@...nel.org>
> >


-- 
Masami Hiramatsu (Google) <mhiramat@...nel.org>