Re: [PATCH 1/1] epoll: remove wrong assert that ep_poll_callback is always called with irqs off

[Roman Penyaev]

On 2019-01-08 16:16, Davidlohr Bueso wrote:
> On 2019-01-08 04:42, Roman Penyaev wrote:
> > What we can do:
> >
> > a) disable irqs if we are not in interrupt.
> > b) revert the patch completely.
> >
> > David, is it really crucial in terms of performance to avoid double
> > local_irq_save() on Xen on this ep_poll_callback() hot path?
>
> Note that such optimizations are also relevant for baremetal, ie: x86
> PUSHF + POPF can be pretty expensive because of insn dependencies.
>
> > For example why not to do the following:
> >
> >   if (!in_interrupt())
> >        local_irq_save(flags);
> >   read_lock(ep->lock);
> >
> > with huge comment explaining performance number.
> >
> > Or just give up and simply revert the original patch completely
> > and always call read_lock_irqsave().
>
> Yeah so the reason why I had done the other epoll lock irq
> optimizations was because they were painfully obvious.
> ep_poll_callback(), however is a different beast, as you've
> encountered. I vote for not shooting ourselves in the foot and just
> dropping this patch -- most large performance benefits will come from
> microbenches anyway.

Then I will send another patch which changes read_lock() on 
read_lock_irqsave(),
since simple revert of the original patch won't work.

Thanks.

--
Roman