Messages in this thread | | | Date | Fri, 20 Jul 2018 13:44:29 -0700 | From | Andrew Morton <> | Subject | Re: [PATCH -next 0/2] fs/epoll: loosen irq safety when possible |
| |
On Fri, 20 Jul 2018 13:05:59 -0700 Davidlohr Bueso <dave@stgolabs.net> wrote:
> On Fri, 20 Jul 2018, Andrew Morton wrote: > > >On Fri, 20 Jul 2018 10:29:54 -0700 Davidlohr Bueso <dave@stgolabs.net> wrote: > > > >> Hi, > >> > >> Both patches replace saving+restoring interrupts when taking the > >> ep->lock (now the waitqueue lock), with just disabling local irqs. > >> This shows immediate performance benefits in patch 1 for an epoll > >> workload running on Xen. > > > >I'm surprised. Is spin_lock_irqsave() significantly more expensive > >than spin_lock_irq()? Relative to all the other stuff those functions > >are doing? If so, how come? Some architectural thing makes > >local_irq_save() much more costly than local_irq_disable()? > > For example, if you compare x86 native_restore_fl() to xen_restore_fl(), > the cost of Xen is much higher. > > And at least considering ep_scan_ready_list(), the lock is taken/released > twice, to deal with the ovflist when the ep->wq.lock is not held. To the > point that it yields measurable results (see patch 1) across incremental > thread counts.
Did you try measuring it on bare hardware?
> > > >> The main concern we need to have with this > >> sort of changes in epoll is the ep_poll_callback() which is passed > >> to the wait queue wakeup and is done very often under irq context, > >> this patch does not touch this call. > > > >Yeah, these changes are scary. For the code as it stands now, and for > >the code as it evolves. > > Yes which is why I've been throwing lots of epoll workloads at it.
I'm sure. It's the "as it evolves" that is worrisome, and has caught us in the past.
> > > >I'd have more confidence if we had some warning mechanism if we run > >spin_lock_irq() when IRQs are disabled, which is probably-a-bug. But > >afaict we don't have that. Probably for good reasons - I wonder what > >they are?
Well ignored ;)
We could open-code it locally. Add a couple of WARN_ON_ONCE(irqs_disabled())? That might need re-benchmarking with Xen but surely just reading the thing isn't too expensive?
| |