Re: [pipe] 3b844826b6: stress-ng.sigio.ops_per_sec -99.3% regression

[Linus Torvalds]

[ Added Colin King, as this looks to be related to stress-sigio.c ]

On Tue, Aug 24, 2021 at 9:22 AM Linus Torvalds
<torvalds@xxxxxxxxxxxxxxxxxxxx> wrote:
>
> On Tue, Aug 24, 2021 at 7:56 AM kernel test robot <oliver.sang@xxxxxxxxx> wrote:
> >
> > FYI, we noticed a -99.3% regression of stress-ng.sigio.ops_per_sec due to commit:
>
> Well, that's bad.
>
> > commit: 3b844826b6c6 ("pipe: avoid unnecessary EPOLLET wakeups under normal loads")
>
> You fix one benchmark, you break another..
>
> What's a bit odd is that this commit basically reverts commit
> 3a34b13a88ca ("pipe: make pipe writes always wake up readers") which
> did *not* result in any kernel test robot report.

Hmm. I have a possible clue.

I suspect this may be timing-related, and I wonder if the kernel test
robot just didn't happen to notice - or report - the other edge of
this performance regression because it ends up being random enough.

In particular, I think stress-sigio.c is subtly buggy, and the "send
SIGIO on every write" may have effectively hidden the bug.

Here's my theory on what's going on, and why it has tghat odd -99.3% regression:

 - the child in stress-sigio.c does multiple writes in quick
succession, while the parent is scheduled away

 - the pipe was empty at the beginning, but we send just *one* SIGIO
for all these writes

 - the parent gets the SIGIO, and goes to stress_sigio_handler(), and
reads the buffer

but the parent doesn't necessarily *empty* the buffer, so it won't get
any more SIGIO signals from subsequent writes.

The reason this is timing-dependent and about luck is because the
*read* size is BUFFER_SIZE (4kB), but the write sizes are BUFFER_SIZE
>> 4 (256 bytes), so 99% of the time if the parent is at all timely,
it will do that big read and empty the buffer.

But if the child ends up able to do more than 16 writes in a row
before the parent wakes up and reads data, the buffer will have more
than 4kB of data in it, and when the parent SIGIO handler happens it
won't empty the data, and so it will never see another SIGIO.

And again - I think this all happened before too (ie before commit
3a34b13a88ca) and so 3b844826b6c6 ("pipe: avoid unnecessary EPOLLET
wakeups under normal loads") isn't a _new_ regression, it just exposed
the bug in the test-case again, and this time the kernel test robot
caught it.

In other words, I think this is very similar to the EPOLLET problem:
user space expected (perhaps not _intentionally_) that SIGIO would
happen over and over again when you add new data, even if the pipe
already had data.

So then commit 1b6b26ae7053 ("pipe: fix and clarify pipe write wakeup
logic") came around back in 2019, and only sent SIGIO when the pipe
buffer status _changed_ (ie went from empty to having data, rather
than "there's new data") and at that point stress-sigio.c became very
fragile.

We could do the same ugly thing for FASYNC that we do for EPOLLET -
make it always fasync on new data, exactly because the previous SIGIO
might not have emptied the buffer completely.

I'm a _bit_ surprised that the kernel test robot didn't catch this
back in 2019, and also didn't report this when commit 3a34b13a88ca
("pipe: make pipe writes always wake up readers") should have sped
things up again, but I guess the "it's very timing-dependent" might
explain it.

Colin, comments?

Anyway, the above blathering is not based on any actual real data. I'm
just looking at stress-sigio.c to see what could be going wrong, and
what would explain the test robot report. It's the only thing I see,
but maybe somebody else sees anything else..

                      Linus