linux-kernel - Re: [PATCH v1 2/5] relayfs: introduce dump of relayfs statistics function

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Message-ID: <CAL+tcoCThLOL1AKPamh2fpeDjYEqvEber0Eddc2ZVxi6VAYJkQ@mail.gmail.com>
Date: Tue, 13 May 2025 18:32:29 +0800
From: Jason Xing <kerneljasonxing@...il.com>
To: Masami Hiramatsu <mhiramat@...nel.org>
Cc: axboe@...nel.dk, rostedt@...dmis.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 v1 2/5] relayfs: introduce dump of relayfs statistics function

Hi Masami,

On Tue, May 13, 2025 at 5:58 PM Masami Hiramatsu <mhiramat@...nel.org> wrote:
>
> Hi Jason,
>
> On Mon, 12 May 2025 10:49:32 +0800
> Jason Xing <kerneljasonxing@...il.com> 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 MUST pass a valid @buf
> > with a valid @len that is required to be larger than RELAY_DUMP_BUF_MAX_LEN
> > to acquire which information indicated by @flags to dump.
> >
> > RELAY_DUMP_BUF_MAX_LEN shows the maximum len of the buffer if users
> > choose to dump all the values.
> >
> > Users can use this buffer to do whatever they expect in their own kernel
> > module, say, print to console/dmesg or write them into the relay buffer.
> >
> > Reviewed-by: Yushan Zhou <katrinzhou@...cent.com>
> > Signed-off-by: Jason Xing <kernelxing@...cent.com>
> > ---
> >  include/linux/relay.h | 10 ++++++++++
> >  kernel/relay.c        | 35 +++++++++++++++++++++++++++++++++++
> >  2 files changed, 45 insertions(+)
> >
> > diff --git a/include/linux/relay.h b/include/linux/relay.h
> > index 022cf11e5a92..7a442c4cbead 100644
> > --- a/include/linux/relay.h
> > +++ b/include/linux/relay.h
> > @@ -31,6 +31,15 @@
> >  /*
> >   * Relay buffer error statistics dump
> >   */
> > +enum {
> > +     RELAY_DUMP_BUF_FULL = (1 << 0),
> > +
> > +     RELAY_DUMP_LAST = RELAY_DUMP_BUF_FULL,
> > +     RELAY_DUMP_MASK = (RELAY_DUMP_LAST - 1) | RELAY_DUMP_LAST
> > +};
> > +
> > +#define RELAY_DUMP_BUF_MAX_LEN 32
> > +
> >  struct rchan_buf_error_stats
> >  {
> >       unsigned int full;              /* counter for buffer full */
> > @@ -170,6 +179,7 @@ extern int relay_late_setup_files(struct rchan *chan,
> >                                 struct dentry *parent);
> >  extern void relay_close(struct rchan *chan);
> >  extern void relay_flush(struct rchan *chan);
> > +extern void relay_dump(struct rchan *chan, char *buf, int len, 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 b5db4aa60da1..0e675a77285c 100644
> > --- a/kernel/relay.c
> > +++ b/kernel/relay.c
> > @@ -810,6 +810,41 @@ void relay_flush(struct rchan *chan)
> >  }
> >  EXPORT_SYMBOL_GPL(relay_flush);
> >
> > +/**
> > + *   relay_dump - dump statistics of the specified channel buffer
> > + *   @chan: the channel
> > + *   @buf: buf to store statistics
> > + *   @len: len of buf to check
> > + *   @flags: select particular information to dump
> > + */
> > +void relay_dump(struct rchan *chan, char *buf, int len, int flags)
> > +{
> > +     unsigned int i, full_counter = 0;
> > +     struct rchan_buf *rbuf;
> > +     int offset = 0;
> > +
> > +     if (!chan || !buf || flags & ~RELAY_DUMP_MASK)
> > +             return;
> > +
> > +     if (len < RELAY_DUMP_BUF_MAX_LEN)
> > +             return;
> > +
> > +     if (chan->is_global) {
> > +             rbuf = *per_cpu_ptr(chan->buf, 0);
> > +             full_counter = rbuf->stats.full;
> > +     } else {
> > +             for_each_possible_cpu(i) {
> > +                     if ((rbuf = *per_cpu_ptr(chan->buf, i)))
> > +                             full_counter += rbuf->stats.full;
> > +     }
> > +
> > +     if (flags & RELAY_DUMP_BUF_FULL)
> > +             offset += snprintf(buf, sizeof(unsigned int), "%u", full_counter);
> > +
> > +     snprintf(buf + offset, 1, "\n");
>
> Is there any reason to return the value as string?
> If it returns a digit value and the caller makes it a string,
> it could be more flexible for other use cases.

Thanks for the feedback.

I will remove the above one as you pointed out in the next revision.
And it seems unnecessary to add '\0' at the end of the buffer like
"*buf = '\0';"?

While at it, I'm thinking if I can change the return value of
relay_dump() to "how many bytes do we actually write into the buffer"?
Does it sound better?

Thanks,
Jason

>
> Thank you,
>
> > +}
> > +EXPORT_SYMBOL_GPL(relay_dump);
> > +
> >  /**
> >   *   relay_file_open - open file op for relay files
> >   *   @inode: the inode
> > --
> > 2.43.5
> >
>
>
> --
> Masami Hiramatsu (Google) <mhiramat@...nel.org>