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[172.10.233.147]) by smtp.gmail.com with ESMTPSA id m2-20020a9d4002000000b005a2678bbc5csm19590ote.24.2022.03.02.12.38.27 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Wed, 02 Mar 2022 12:38:28 -0800 (PST) Date: Wed, 2 Mar 2022 12:38:00 -0800 (PST) From: Hugh Dickins X-X-Sender: hugh@ripple.anvils To: Michal Hocko cc: Andrew Morton , cgel.zte@gmail.com, naoya.horiguchi@nec.com, minchan@kernel.org, hannes@cmpxchg.org, rogerq@kernel.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org, guo.ziliang@zte.com.cn, Zeal Robot , Ran Xiaokai , Jiang Xuexin , Yang Yang Subject: Re: [PATCH linux-next] mm: swap: get rid of deadloop in swapin readahead In-Reply-To: Message-ID: <94de248f-94fa-91d-eb80-2ea75548282d@google.com> References: <20220221111749.1928222-1-cgel.zte@gmail.com> <20220228073315.c855e15b108089187a40d2b0@linux-foundation.org> MIME-Version: 1.0 Content-Type: text/plain; charset=US-ASCII X-Rspam-User: X-Rspamd-Server: rspam12 X-Rspamd-Queue-Id: 6072714000C X-Stat-Signature: txbs5mezyks4i33u9wa7xkz9xb5qms1a Authentication-Results: imf26.hostedemail.com; dkim=pass header.d=google.com header.s=20210112 header.b=NDC3WLqJ; spf=pass (imf26.hostedemail.com: domain of hughd@google.com designates 209.85.167.176 as permitted sender) smtp.mailfrom=hughd@google.com; dmarc=pass (policy=reject) header.from=google.com X-HE-Tag: 1646253510-688506 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: On Wed, 2 Mar 2022, Michal Hocko wrote: > > I might be really missing something but I really do not see how is this > any different from the page allocator path which only does cond_resched > as well (well, except for throttling but that might just not trigger). > Or other paths which just do cond_resched while waiting for a progress > somewhere else. > > Not that I like this situation but !PREEMPT kernel with RT priority > tasks is rather limited and full of potential priblems IMHO. As I said in previous mail, I have really not given this as much thought this time as I did in the 2018 mail thread linked there; but have seen that it behaves more badly than I had imagined, in any preemptive kernel - no need for RT. We just don't have the stats to show when this code here spins waiting on code elsewhere that is sleeping. I think the difference from most cond_resched() places is that swapin is trying to collect together several factors with minimal locking, and we should have added preempt_disable()s when preemption was invented. But it's only swap so we didn't notice. Hugh