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

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Message-ID: <CAEf4BzaKcrKXsdEu9Jo6=QAgn+SX3QxjmAt3vQEQ46UOkLh5eg@mail.gmail.com>
Date: Wed, 21 Aug 2024 09:38:44 -0700
From: Andrii Nakryiko <andrii.nakryiko@...il.com>
To: Andrii Nakryiko <andrii@...nel.org>
Cc: linux-trace-kernel@...r.kernel.org, rostedt@...dmis.org, 
	mhiramat@...nel.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 Tue, Aug 13, 2024 at 1:34 PM 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.
>
> 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(-)
>

Is there anything else I'm expected to do about this patch? If not,
can this please be applied? Thanks!

> 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
>