Messages in this thread | | | Date | Tue, 08 Jan 2019 17:07:23 +0100 | From | Roman Penyaev <> | Subject | Re: [PATCH 1/1] epoll: remove wrong assert that ep_poll_callback is always called with irqs off |
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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
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