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b=d7D2CZnn1vLpvysuU2ME7JBu80T75XHsBcmams9VD7htZCi+hF4Mlb+rWuP6PeuYd nTEwmGb2/1RjYjY0YRwVKpzCimtihJdMevh3c1KaXTGRJtqXzBRK1DRXHWmphT2zqo 0GfS2REKDcoRTlppPMsv4t3Cwe9Q6cMQUsMyGZZtz8OA4EL2dfJ4+HvaVu8l2h8EsL 5S22obIng9rHhOS/10l92QiJbqB1FrtUWmimFlx5qxIfzpVX5A3fLF0KKc8j4z6a/v 5Fq8vPqrQ4P3NA3XINDnmcl3o4EBbn9goW1BVV8dTNn6QnDR4Q76sA0h9lXc6RjVB3 UW5qou2yLteHw== Message-ID: Date: Thu, 27 Jun 2024 21:18:35 +0200 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH V3 2/2] cgroup/rstat: Avoid thundering herd problem by kswapd across NUMA nodes To: Shakeel Butt , Yosry Ahmed Cc: tj@kernel.org, cgroups@vger.kernel.org, hannes@cmpxchg.org, lizefan.x@bytedance.com, longman@redhat.com, kernel-team@cloudflare.com, linux-mm@kvack.org, linux-kernel@vger.kernel.org References: <171943667611.1638606.4158229160024621051.stgit@firesoul> <171943668946.1638606.1320095353103578332.stgit@firesoul> Content-Language: en-US From: Jesper Dangaard Brouer In-Reply-To: Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit X-Rspamd-Server: rspam03 X-Rspam-User: X-Rspamd-Queue-Id: 9E5C9C0005 X-Stat-Signature: tbfbm49mkkyfkepdtpzbyeqwag7ywi31 X-HE-Tag: 1719515923-966252 X-HE-Meta: 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 V+H11+pJ Juix5RLyfcWNpZV3wgUM/9a+5Og+ZdQf+CHO7TD7EziNk5XWYkVGNQocWzxslcMlPzWX2tPRKScpwYc3zKKyhPJpqmZdeMIJFhRYpzv8Z07b7rocRc4oN3JtrOtWbsoHdxngUrYbMsWKFrBAD+aNKQaSHTwS0K8OG8e7ut9usv4MkjSSJb1cpy8RnsRZBFgPzboChaSptb0m2Z2B7IicWHbLS/4EsKVllHdeIpKY/KwigMfiRYGuMfxS1Mkl7+jiUrZMPpsOouSQ/dDlKuV6meV/9+w== X-Bogosity: Ham, tests=bogofilter, spamicity=0.102014, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 27/06/2024 20.45, Shakeel Butt wrote: > On Thu, Jun 27, 2024 at 04:32:03AM GMT, Yosry Ahmed wrote: >> On Thu, Jun 27, 2024 at 3:33 AM Yosry Ahmed wrote: > [...] >>> >>> The reason why I suggested that the completion live in struct cgroup >>> is because there is a chance here that the flush completes and another >>> irrelevant flush starts between reading cgrp_rstat_ongoing_flusher and >>> calling wait_for_completion_interruptible_timeout(). > I didn't add this per cgroup because I fear the race of adding a wait_for_completion on a cgroup that gets stuck there, but looking at the code the completion API should be able to avoid this. > Yes this can happen if flusher for irrelevant cgroup calls > reinit_completion() while the initial flusher was just about to call > wait_for_completion_interruptible_timeout(). > Restoring two main functions to assist reviewer seeing the race: On 26/06/2024 23.18, Jesper Dangaard Brouer wrote: > +#define MAX_WAIT msecs_to_jiffies(100) > +/* Trylock helper that also checks for on ongoing flusher */ > +static bool cgroup_rstat_trylock_flusher(struct cgroup *cgrp) > +{ > +retry: > + bool locked = __cgroup_rstat_trylock(cgrp, -1); > + if (!locked) { > + struct cgroup *cgrp_ongoing; > + > + /* Lock is contended, lets check if ongoing flusher is > + * taking care of this, if we are a descendant. > + */ > + cgrp_ongoing = READ_ONCE(cgrp_rstat_ongoing_flusher); > + if (!cgrp_ongoing) > + goto retry; > + Long wait/race here, can cause us to see an out-dated cgrp_ongoing. And then another CPU manage to reach reinit_completion() below, before execution continues here. > + if (cgroup_is_descendant(cgrp, cgrp_ongoing)) { > + wait_for_completion_interruptible_timeout( > + &cgrp_rstat_flusher_done, MAX_WAIT); > + > + return false; > + } > + __cgroup_rstat_lock(cgrp, -1, false); > + } > + /* Obtained lock, record this cgrp as the ongoing flusher */ > + reinit_completion(&cgrp_rstat_flusher_done); > + WRITE_ONCE(cgrp_rstat_ongoing_flusher, cgrp); > + > + return true; /* locked */ > +} > + > +static void cgroup_rstat_unlock_flusher(struct cgroup *cgrp) > +{ > + WRITE_ONCE(cgrp_rstat_ongoing_flusher, NULL); > + complete_all(&cgrp_rstat_flusher_done); > + __cgroup_rstat_unlock(cgrp, -1); > +} >>> >>> This will cause the caller to wait for an irrelevant flush, which may >>> be fine because today the caller would wait for the lock anyway. Just >>> mentioning this in case you think this may happen enough to be a >>> problem. Yes, it would wait for an irrelevant flush. >> >> Actually, I think this can happen beyond the window I described above. >> I think it's possible that a thread waits for the flush, then gets >> woken up when complete_all() is called, but another flusher calls >> reinit_completion() immediately. The woken up thread will observe >> completion->done == 0 and go to sleep again. > > I don't think it will go to sleep again as there is no retry. > >> >> I think most of these cases can be avoided if we make the completion >> per cgroup. It is still possible to wait for more flushes than >> necessary, but only if they are for the same cgroup. > > Yeah, per-cgroup completion would avoid the problem of waiting for > irrelevant flush. Great, I will code up a version with per-cgroup completion. --Jesper