Message-ID: <CAL+tcoAGCdowY14QL7HEqbW3ewAJi0OXpWNVkbqq9cM6xRmLkg@mail.gmail.com>
Date: Tue, 13 May 2025 21:46:25 +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

On Tue, May 13, 2025 at 9:22 PM Masami Hiramatsu <mhiramat@...nel.org> wrote:
>
> On Tue, 13 May 2025 18:32:29 +0800
> Jason Xing <kerneljasonxing@...il.com> wrote:
>
> > 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';"?
>
> Sorry, I think you missed my point. I meant it should be
>
> /* Return the number of fullfilled buffer in the channel */
> size_t relay_full(struct rchan *chan);
>
> And keep the snprintf() in the blk_dropped_read() because that function
> is responsible for formatting the output.

Oh, sorry, it's not what I want because (please see patch [4/5] [1])
relay_dump() works to print various statistics of the buffer. In this
patch, 'full' counter is the first one.

[1]: https://lore.kernel.org/all/20250512024935.64704-5-kerneljasonxing@gmail.com/

>
> static ssize_t blk_dropped_read(struct file *filp, char __user *buffer,
>                                 size_t count, loff_t *ppos)
> {
>         struct blk_trace *bt = filp->private_data;
>         char buf[16];
>
>         snprintf(buf, sizeof(buf), "%zu\n", relay_full(bt->rchan));
>
>         return simple_read_from_buffer(buffer, count, ppos, buf, strlen(buf));
> }
>
> But the question is that what blk_subbuf_start_callback() counts
> is really equal to what the relay_full() counts, because it seems
> relay_full() just returns the current status of the channel, but
> bt->dropped is the accumlated number of dropping events.
>
> This means that if the client (reader) reads the subbufs the
> number of full subbufs will be decreased, but the number of
> dropped event does not change.

At least in this series, I didn't give the 'full' counter any chance
to decrement, which means it's compatible with blktrace, unless
__relay_reset() is triggered :)

While discussing with you on this point, I suddenly recalled that in
some network drivers, they implemented 'wake' and 'stop' as a pair to
know what the current status of a certain queue is. I think I can add
a similar thing to the buffer about 1) how many times are the buffer
full, 2) how many times does the buffer get rid of being full.

Thanks,
Jason

>
> Can you recheck it?
>
> Thank you,
>
> >
> > 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>
>
>
> --
> Masami Hiramatsu (Google) <mhiramat@...nel.org>

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