From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mx2.suse.de ([195.135.220.15]:59020 "EHLO mx1.suse.de" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1727484AbfAHPQM (ORCPT ); Tue, 8 Jan 2019 10:16:12 -0500 MIME-Version: 1.0 Content-Type: text/plain; charset=US-ASCII; format=flowed Content-Transfer-Encoding: 7bit Date: Tue, 08 Jan 2019 07:16:10 -0800 From: Davidlohr Bueso To: Roman Penyaev Cc: Jason Baron , Al Viro , Andrew Morton , Linus Torvalds , linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH 1/1] epoll: remove wrong assert that ep_poll_callback is always called with irqs off In-Reply-To: References: <20190108100121.20247-1-rpenyaev@suse.de> Message-ID: <725cdce88418c2ec62ef6014d388dbeb@suse.de> Sender: linux-fsdevel-owner@vger.kernel.org List-ID: 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. Thanks, Davidlohr