linux-kernel - Re: [PATCH 1/2] ring-buffer: Replace rb_time_cmpxchg() with rb_time_cmp_and

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Date: Mon, 18 Dec 2023 23:24:55 +0900
From: Masami Hiramatsu (Google) <mhiramat@...nel.org>
To: Steven Rostedt <rostedt@...dmis.org>
Cc: linux-kernel@...r.kernel.org, linux-trace-kernel@...r.kernel.org, Masami
 Hiramatsu <mhiramat@...nel.org>, Mark Rutland <mark.rutland@....com>,
 Mathieu Desnoyers <mathieu.desnoyers@...icios.com>, Andrew Morton
 <akpm@...ux-foundation.org>, Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCH 1/2] ring-buffer: Replace rb_time_cmpxchg() with
 rb_time_cmp_and_update()

On Fri, 15 Dec 2023 11:55:13 -0500
Steven Rostedt <rostedt@...dmis.org> wrote:

> From: "Steven Rostedt (Google)" <rostedt@...dmis.org>
> 
> There's only one place that performs a 64-bit cmpxchg for the timestamp
> processing. The cmpxchg is only to set the write_stamp equal to the
> before_stamp, and if it doesn't get set, then the next event will simply
> be forced to add an absolute timestamp.
> 
> Given that 64-bit cmpxchg is expensive on 32-bit, and the current
> workaround uses 3 consecutive 32-bit cmpxchg doesn't make it any faster.
> It's best to just not do the cmpxchg as a simple compare works for the
> accuracy of the timestamp. The only thing that will happen without the
> cmpxchg is the prepended absolute timestamp on the next event which is not
> that big of a deal as the path where this happens is seldom hit because it
> requires an interrupt to happen between a few lines of code that also
> writes an event into the same buffer.
> 
> With this change, the 32-bit rb_time_t workaround can be removed.
> 

Hmm, but this patch itself is just moving rb_time_cmpxchg() in the new
rb_time_cmp_and_update() function. The actual change has been done
in the next patch. I think there is no reason to split this from the
second one...

Isn't this part actual change?

>  static bool rb_time_cmp_and_update(rb_time_t *t, u64 expect, u64 set)
>  {
> -	return rb_time_cmpxchg(t, expect, set);
> +#ifdef RB_TIME_32
> +	return expect == READ_ONCE(t->time);
> +#else
> +	return local64_try_cmpxchg(&t->time, &expect, set);
> +#endif
>  }

Thank you,

> Signed-off-by: Steven Rostedt (Google) <rostedt@...dmis.org>
> ---
>  kernel/trace/ring_buffer.c | 23 ++++++++++++++++++++---
>  1 file changed, 20 insertions(+), 3 deletions(-)
> 
> diff --git a/kernel/trace/ring_buffer.c b/kernel/trace/ring_buffer.c
> index 1a26b3a1901f..c6842a4331a9 100644
> --- a/kernel/trace/ring_buffer.c
> +++ b/kernel/trace/ring_buffer.c
> @@ -762,6 +762,15 @@ static bool rb_time_cmpxchg(rb_time_t *t, u64 expect, u64 set)
>  }
>  #endif
>  
> +/*
> + * Returns true if t == expect, and if possible will update with set,
> + * but t is not guaranteed to be updated even if this returns true
> + */
> +static bool rb_time_cmp_and_update(rb_time_t *t, u64 expect, u64 set)
> +{
> +	return rb_time_cmpxchg(t, expect, set);
> +}
> +
>  /*
>   * Enable this to make sure that the event passed to
>   * ring_buffer_event_time_stamp() is not committed and also
> @@ -3622,9 +3631,17 @@ __rb_reserve_next(struct ring_buffer_per_cpu *cpu_buffer,
>  		barrier();
>   /*E*/		if (write == (local_read(&tail_page->write) & RB_WRITE_MASK) &&
>  		    info->after < ts &&
> -		    rb_time_cmpxchg(&cpu_buffer->write_stamp,
> -				    info->after, ts)) {
> -			/* Nothing came after this event between C and E */
> +		    rb_time_cmp_and_update(&cpu_buffer->write_stamp,
> +					   info->after, ts)) {
> +			/*
> +			 * Nothing came after this event between C and E it is
> +			 * safe to use info->after for the delta.
> +			 * The above rb_time_cmp_and_update() may or may not
> +			 * have updated the write_stamp. If it did not then
> +			 * the next event will simply add an absolute timestamp
> +			 * as the write_stamp will be different than the
> +			 * before_stamp.
> +			 */
>  			info->delta = ts - info->after;
>  		} else {
>  			/*
> -- 
> 2.42.0
> 
> 


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