Re: [PATCH v2] perf bench: add --write-size option to sched pipe

[Namhyung Kim]

On Tue, May 26, 2026 at 06:55:33PM +0100, Breno Leitao wrote:
> hello Namhyung,
> 
> On Mon, May 25, 2026 at 09:17:45PM +0000, Namhyung Kim wrote:
> > On Thu, May 21, 2026 at 09:15:37AM -0700, Breno Leitao wrote:
> > > +		ret = write(td->pipe_write, td->buf, write_size);
> > > +		BUG_ON(ret < 0 || (unsigned int)ret != write_size);
> > >  		ret = read_pipe(td);
> > > -		BUG_ON(ret != sizeof(int));
> > > +		BUG_ON(ret < 0 || (unsigned int)ret != write_size);
> > 
> > Is it possible to return smaller values than required due to signal or
> > something?
> 
> I tested this on a VM with the patch applied, running the ping-pong with
> write_size from 4 B up to 1 MiB, including multi-process and single-CPU
> contention runs (millions of iterations). I observed zero short reads or short
> writes.
> 
> Looking at fs/pipe.c, this matches the kernel behavior for this access pattern:
> the bench has a single writer per pipe and the pipe capacity is set to
> write_size, so the writer never has to sleep mid-write, and the kernel does a
> sync wake of the reader only after the full payload is queued.
> 
> EINTR is also not reachable today because the bench installs no signal
> handlers and nothing in the call chain delivers signals to the workers
> 
> At the same time, I understand the concern and If you'd still prefer defensive
> handling (short-return loop + EINTR retry) I can add it in v3 — happy to either
> way.

Thanks for looking into it.  I think it's better to go defensive for any
unexpected behavior or changes.

Thanks,
Namhyung