Re: [PATCH for 5.9 1/3] futex: introduce FUTEX_SWAP operation

[Waiman Long]

On 7/23/20 8:25 PM, Peter Oskolkov wrote:
> On Thu, Jul 23, 2020 at 4:28 AM Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
>
> Thanks a lot for your comments, Peter! My answers below.
>
> > On Wed, Jul 22, 2020 at 04:45:36PM -0700, Peter Oskolkov wrote:
> > > This patchset is the first step to open-source this work. As explained
> > > in the linked pdf and video, SwitchTo API has three core operations: wait,
> > > resume, and swap (=switch). So this patchset adds a FUTEX_SWAP operation
> > > that, in addition to FUTEX_WAIT and FUTEX_WAKE, will provide a foundation
> > > on top of which user-space threading libraries can be built.
> > The PDF and video can go pound sand; you get to fully explain things
> > here.
> Will do. Should I expand the cover letter or the commit message? (I'll probably
> split the first patch into two in the latter case).

You should put it mostly in the commit message which will be part of the 
git log history. The cover letter, on the other hand, is not part of the 
git log.


> > What worries me is how FUTEX_SWAP would interact with the future
> > FUTEX_LOCK / FUTEX_UNLOCK. When we implement pthread_mutex with those,
> > there's very few WAIT/WAKE left.
> [+cc Waiman Long]
>
> I've looked through the latest FUTEX_LOCK patchset I could find (
> https://lore.kernel.org/patchwork/cover/772643/ and related), and it seems
> that FUTEX_SWAP and FUTEX_LOCK/FUTEX_UNLOCK patchsets
> address the same issue (slow wakeups) but for different use cases:
>
> FUTEX_LOCK/FUTEX_UNLOCK uses spinning and lock stealing to
> improve futex wake/wait performance in high contention situations;
> FUTEX_SWAP is designed to be used for fast context switching with
> _no_ contention by design: the waker that is going to sleep, and the wakee
> are using different futexes; the userspace will have a futex per thread/task,
> and when needed the thread/task will either simply sleep on its futex,
> or context switch (=FUTEX_SWAP) into a different thread/task.

I agree that it is a different use case. I just hope that you keep the 
possible future extension to support FUTEX_LOCK/UNLOCK in mind so that 
they won't conflict with each other.

Cheers,
Longman