linux-kernel - Re: [PATCH v2 2/4] relayfs: introduce dump of relayfs statistics function

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Message-ID: <CAL+tcoB7NBWLQgQqYX8QjJcvpJmDB39iZMrZWZA07PF6ELYfZA@mail.gmail.com>
Date: Fri, 16 May 2025 07:24:47 +0800
From: Jason Xing <kerneljasonxing@...il.com>
To: Jens Axboe <axboe@...nel.dk>
Cc: rostedt@...dmis.org, mhiramat@...nel.org, mathieu.desnoyers@...icios.com, 
	akpm@...ux-foundation.org, linux-kernel@...r.kernel.org, 
	linux-block@...r.kernel.org, linux-trace-kernel@...r.kernel.org, 
	Jason Xing <kernelxing@...cent.com>, Yushan Zhou <katrinzhou@...cent.com>
Subject: Re: [PATCH v2 2/4] relayfs: introduce dump of relayfs statistics function

On Fri, May 16, 2025 at 2:19 AM Jens Axboe <axboe@...nel.dk> wrote:
>
> On 5/15/25 12:16 AM, Jason Xing wrote:
> > From: Jason Xing <kernelxing@...cent.com>
> >
> > In this version, only support dumping the counter for buffer full and
> > implement the framework of how it works.
> >
> > Users can pass certain flag to fetch what field/statistics they expect
> > to know. Each time it only returns one result. So do not pass multiple
> > flags.
> >
> > Reviewed-by: Yushan Zhou <katrinzhou@...cent.com>
> > Signed-off-by: Jason Xing <kernelxing@...cent.com>
> > ---
> > v2
> > 1. refactor relay_dump() and make it only return a pure size_t result
> > of the value that users specifies.
> > 2. revise the commit log.
> > ---
> >  include/linux/relay.h |  7 +++++++
> >  kernel/relay.c        | 31 +++++++++++++++++++++++++++++++
> >  2 files changed, 38 insertions(+)
> >
> > diff --git a/include/linux/relay.h b/include/linux/relay.h
> > index ce7a1b396872..3fb285716e34 100644
> > --- a/include/linux/relay.h
> > +++ b/include/linux/relay.h
> > @@ -31,6 +31,12 @@
> >  /*
> >   * Relay buffer statistics dump
> >   */
> > +enum {
> > +     RELAY_DUMP_BUF_FULL = (1 << 0),
> > +
> > +     RELAY_DUMP_LAST = RELAY_DUMP_BUF_FULL,
> > +};
> > +
> >  struct rchan_buf_stats
> >  {
> >       unsigned int full_count;        /* counter for buffer full */
> > @@ -167,6 +173,7 @@ struct rchan *relay_open(const char *base_filename,
> >                        void *private_data);
> >  extern void relay_close(struct rchan *chan);
> >  extern void relay_flush(struct rchan *chan);
> > +extern size_t relay_dump(struct rchan *chan, int flags);
> >  extern void relay_subbufs_consumed(struct rchan *chan,
> >                                  unsigned int cpu,
> >                                  size_t consumed);
> > diff --git a/kernel/relay.c b/kernel/relay.c
> > index eb3f630f3896..f47fc750e559 100644
> > --- a/kernel/relay.c
> > +++ b/kernel/relay.c
> > @@ -701,6 +701,37 @@ void relay_flush(struct rchan *chan)
> >  }
> >  EXPORT_SYMBOL_GPL(relay_flush);
> >
> > +/**
> > + *   relay_dump - dump channel buffer statistics
> > + *   @chan: the channel
> > + *   @flags: select particular information to dump
> > + *
> > + *   Returns the count of certain field that caller specifies.
> > + */
> > +size_t relay_dump(struct rchan *chan, int flags)
> > +{
> > +     unsigned int i, count = 0;
> > +     struct rchan_buf *rbuf;
> > +
> > +     if (!chan || flags > RELAY_DUMP_LAST)
> > +             return 0;
> > +
> > +     if (chan->is_global) {
> > +             rbuf = *per_cpu_ptr(chan->buf, 0);
> > +             if (flags & RELAY_DUMP_BUF_FULL)
> > +                     count = rbuf->stats.full_count;
> > +     } else {
> > +             for_each_online_cpu(i) {
> > +                     if ((rbuf = *per_cpu_ptr(chan->buf, i)))
> > +                             if (flags & RELAY_DUMP_BUF_FULL)
> > +                                     count += rbuf->stats.full_count;
>
> Kernel tends to avoid the rolled-into-one assignment and check, it's
> easy to misread. This:
>
>         rbuf = *per_cpu_ptr(chan->buf, i);
>         if (rbuf && flags & RELAY_DUMP_BUF_FULL)
>                 count += rbuf->stats.full_count;
>
> reads much easier. IMHO.

Agreed. Thanks. I will rewrite this part in V3 :)

Thanks,
Jason

>
> --
> Jens Axboe