Re: [PATCH v11 8/8] vhost: use vhost_tasks for worker threads

From: Jens Axboe
Date: Mon May 15 2023 - 11:52:18 EST

On 5/15/23 9:44?AM, Linus Torvalds wrote:
> On Mon, May 15, 2023 at 7:23?AM Christian Brauner <brauner@xxxxxxxxxx> wrote:
>> So I think we will be able to address (1) and (2) by making vhost tasks
>> proper threads and blocking every signal except for SIGKILL and SIGSTOP
>> and then having vhost handle get_signal() - as you mentioned - the same
>> way io uring already does. We should also remove the ingore_signals
>> thing completely imho. I don't think we ever want to do this with user
>> workers.
> Right. That's what IO_URING does:
> if (args->io_thread) {
> /*
> * Mark us an IO worker, and block any signal that isn't
> * fatal or STOP
> */
> p->flags |= PF_IO_WORKER;
> siginitsetinv(&p->blocked, sigmask(SIGKILL)|sigmask(SIGSTOP));
> }
> and I really think that vhost should basically do exactly what io_uring does.
> Not because io_uring fundamentally got this right - but simply because
> io_uring had almost all the same bugs (and then some), and what the
> io_uring worker threads ended up doing was to basically zoom in on
> "this works".
> And it zoomed in on it largely by just going for "make it look as much
> as possible as a real user thread", because every time the kernel
> thread did something different, it just caused problems.

This is exactly what I told Christian in a private chat too - we went
through all of that, and this is what works. KISS.

> So I think the patch should just look something like the attached.
> Mike, can you test this on whatever vhost test-suite?

Seems like that didn't get attached...

> I did consider getting rid of ".ignore_signals" entirely, and instead
> just keying the "block signals" behavior off the ".user_worker" flag.
> But this approach doesn't seem wrong either, and I don't think it's
> wrong to make the create_io_thread() function say that
> ".ignore_signals = 1" thing explicitly, rather than key it off the
> ".io_thread" flag.
> Jens/Christian - comments?
> Slightly related to this all: I think vhost should also do
> CLONE_FILES, and get rid of the whole ".no_files" thing. Again, if
> vhost doesn't use any files, it shouldn't matter, and looking
> different just to be different is wrong. But if vhost doesn't use any
> files, the current situation shouldn't be a bug either.

Only potential downside is that it does make file references more
expensive for other syscalls, since you now have a shared file table.
But probably not something to worry about here?

Jens Axboe