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Wed, 13 Nov 2024 10:07:35 +0000 (UTC) Received: from dovecot-director2.suse.de ([2a07:de40:b281:106:10:150:64:167]) by imap1.dmz-prg2.suse.org with ESMTPSA id lja8K2d6NGeeHgAAD6G6ig (envelope-from ); Wed, 13 Nov 2024 10:07:35 +0000 Received: by quack3.suse.cz (Postfix, from userid 1000) id 643DBA08D0; Wed, 13 Nov 2024 11:07:35 +0100 (CET) Date: Wed, 13 Nov 2024 11:07:35 +0100 From: Jan Kara To: Jim Zhao Cc: jack@suse.cz, akpm@linux-foundation.org, linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org, linux-mm@kvack.org, willy@infradead.org Subject: Re: [PATCH] mm/page-writeback: Raise wb_thresh to prevent write blocking with strictlimit Message-ID: <20241113100735.4jafa56p4td66z7a@quack3> References: <20241108220215.s27rziym6mn5nzv4@quack3> <20241112084539.702485-1-jimzhao.ai@gmail.com> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <20241112084539.702485-1-jimzhao.ai@gmail.com> X-Spam-Score: -2.30 X-Spamd-Result: default: False [-2.30 / 50.00]; BAYES_HAM(-3.00)[100.00%]; SUSPICIOUS_RECIPS(1.50)[]; NEURAL_HAM_LONG(-1.00)[-0.999]; MID_RHS_NOT_FQDN(0.50)[]; NEURAL_HAM_SHORT(-0.20)[-0.999]; MIME_GOOD(-0.10)[text/plain]; RCVD_VIA_SMTP_AUTH(0.00)[]; TAGGED_RCPT(0.00)[]; MISSING_XM_UA(0.00)[]; FREEMAIL_TO(0.00)[gmail.com]; ARC_NA(0.00)[]; MIME_TRACE(0.00)[0:+]; RCPT_COUNT_SEVEN(0.00)[7]; FREEMAIL_ENVRCPT(0.00)[gmail.com]; TO_DN_SOME(0.00)[]; FROM_EQ_ENVFROM(0.00)[]; FROM_HAS_DN(0.00)[]; RCVD_COUNT_THREE(0.00)[3]; TO_MATCH_ENVRCPT_ALL(0.00)[]; FUZZY_BLOCKED(0.00)[rspamd.com]; RCVD_TLS_LAST(0.00)[]; DKIM_SIGNED(0.00)[suse.cz:s=susede2_rsa,suse.cz:s=susede2_ed25519]; DBL_BLOCKED_OPENRESOLVER(0.00)[imap1.dmz-prg2.suse.org:helo,suse.com:email] X-Spam-Flag: NO X-Spam-Level: On Tue 12-11-24 16:45:39, Jim Zhao wrote: > > On Fri 08-11-24 11:19:49, Jim Zhao wrote: > > > > On Wed 23-10-24 18:00:32, Jim Zhao wrote: > > > > > With the strictlimit flag, wb_thresh acts as a hard limit in > > > > > balance_dirty_pages() and wb_position_ratio(). When device write > > > > > operations are inactive, wb_thresh can drop to 0, causing writes to > > > > > be blocked. The issue occasionally occurs in fuse fs, particularly > > > > > with network backends, the write thread is blocked frequently during > > > > > a period. To address it, this patch raises the minimum wb_thresh to a > > > > > controllable level, similar to the non-strictlimit case. > > > > > > > > > > Signed-off-by: Jim Zhao > > > > > > > > ... > > > > > > > > > + /* > > > > > + * With strictlimit flag, the wb_thresh is treated as > > > > > + * a hard limit in balance_dirty_pages() and wb_position_ratio(). > > > > > + * It's possible that wb_thresh is close to zero, not because > > > > > + * the device is slow, but because it has been inactive. > > > > > + * To prevent occasional writes from being blocked, we raise wb_thresh. > > > > > + */ > > > > > + if (unlikely(wb->bdi->capabilities & BDI_CAP_STRICTLIMIT)) { > > > > > + unsigned long limit = hard_dirty_limit(dom, dtc->thresh); > > > > > + u64 wb_scale_thresh = 0; > > > > > + > > > > > + if (limit > dtc->dirty) > > > > > + wb_scale_thresh = (limit - dtc->dirty) / 100; > > > > > + wb_thresh = max(wb_thresh, min(wb_scale_thresh, wb_max_thresh / 4)); > > > > > + } > > > > > > > > What you propose makes sense in principle although I'd say this is mostly a > > > > userspace setup issue - with strictlimit enabled, you're kind of expected > > > > to set min_ratio exactly if you want to avoid these startup issues. But I > > > > tend to agree that we can provide a bit of a slack for a bdi without > > > > min_ratio configured to ramp up. > > > > > > > > But I'd rather pick the logic like: > > > > > > > > /* > > > > * If bdi does not have min_ratio configured and it was inactive, > > > > * bump its min_ratio to 0.1% to provide it some room to ramp up. > > > > */ > > > > if (!wb_min_ratio && !numerator) > > > > wb_min_ratio = min(BDI_RATIO_SCALE / 10, wb_max_ratio / 2); > > > > > > > > That would seem like a bit more systematic way than the formula you propose > > > > above... > > > > > > Thanks for the advice. > > > Here's the explanation of the formula: > > > 1. when writes are small and intermittent，wb_thresh can approach 0, not > > > just 0, making the numerator value difficult to verify. > > > > I see, ok. > > > > > 2. The ramp-up margin, whether 0.1% or another value, needs > > > consideration. > > > I based this on the logic of wb_position_ratio in the non-strictlimit > > > scenario: wb_thresh = max(wb_thresh, (limit - dtc->dirty) / 8); It seems > > > provides more room and ensures ramping up within a controllable range. > > > > I see, thanks for explanation. So I was thinking how to make the code more > > consistent instead of adding another special constant and workaround. What > > I'd suggest is: > > > > 1) There's already code that's supposed to handle ramping up with > > strictlimit in wb_update_dirty_ratelimit(): > > > > /* > > * For strictlimit case, calculations above were based on wb counters > > * and limits (starting from pos_ratio = wb_position_ratio() and up to > > * balanced_dirty_ratelimit = task_ratelimit * write_bw / dirty_rate). > > * Hence, to calculate "step" properly, we have to use wb_dirty as > > * "dirty" and wb_setpoint as "setpoint". > > * > > * We rampup dirty_ratelimit forcibly if wb_dirty is low because > > * it's possible that wb_thresh is close to zero due to inactivity > > * of backing device. > > */ > > if (unlikely(wb->bdi->capabilities & BDI_CAP_STRICTLIMIT)) { > > dirty = dtc->wb_dirty; > > if (dtc->wb_dirty < 8) > > setpoint = dtc->wb_dirty + 1; > > else > > setpoint = (dtc->wb_thresh + dtc->wb_bg_thresh) / 2; > > } > > > > Now I agree that increasing wb_thresh directly is more understandable and > > transparent so I'd just drop this special case. > > yes, I agree. > > > 2) I'd just handle all the bumping of wb_thresh in a single place instead > > of having is spread over multiple places. So __wb_calc_thresh() could have > > a code like: > > > > wb_thresh = (thresh * (100 * BDI_RATIO_SCALE - bdi_min_ratio)) / (100 * BDI_RATIO_SCALE) > > wb_thresh *= numerator; > > wb_thresh = div64_ul(wb_thresh, denominator); > > > > wb_min_max_ratio(dtc->wb, &wb_min_ratio, &wb_max_ratio); > > > > wb_thresh += (thresh * wb_min_ratio) / (100 * BDI_RATIO_SCALE); > > limit = hard_dirty_limit(dtc_dom(dtc), dtc->thresh); > > /* > > * It's very possible that wb_thresh is close to 0 not because the > > * device is slow, but that it has remained inactive for long time. > > * Honour such devices a reasonable good (hopefully IO efficient) > > * threshold, so that the occasional writes won't be blocked and active > > * writes can rampup the threshold quickly. > > */ > > if (limit > dtc->dirty) > > wb_thresh = max(wb_thresh, (limit - dtc->dirty) / 8); > > if (wb_thresh > (thresh * wb_max_ratio) / (100 * BDI_RATIO_SCALE)) > > wb_thresh = thresh * wb_max_ratio / (100 * BDI_RATIO_SCALE); > > > > and we can drop the bumping from wb_position)_ratio(). This way have the > > wb_thresh bumping in a single logical place. Since we still limit wb_tresh > > with max_ratio, untrusted bdis for which max_ratio should be configured > > (otherwise they can grow amount of dirty pages upto global treshold anyway) > > are still under control. > > > > If we really wanted, we could introduce a different bumping in case of > > strictlimit, but at this point I don't think it is warranted so I'd leave > > that as an option if someone comes with a situation where this bumping > > proves to be too aggressive. > > Thank you, this is very helpful. And I have 2 concerns: > > 1. > In the current non-strictlimit logic, wb_thresh is only bumped within > wb_position_ratio() for calculating pos_ratio, and this bump isn’t > restricted by max_ratio. I’m unsure if moving this adjustment to > __wb_calc_thresh() would effect existing behavior. Would it be possible > to keep the current logic for non-strictlimit case? You are correct that current bumping is not affected by max_ratio and that is actually a bug. wb_thresh should never exceed what is corresponding to the configured max_ratio. Furthermore in practical configurations I don't think the max_ratio limiting will actually make a big difference because bumping should happen when wb_thresh is really low. So for consistency I would apply it also to the non-strictlimit case. > 2. Regarding the formula: > wb_thresh = max(wb_thresh, (limit - dtc->dirty) / 8); > > Consider a case: > With 100 fuse devices(with high max_ratio) experiencing high writeback > delays, the pages being written back are accounted in NR_WRITEBACK_TEMP, > not dtc->dirty. As a result, the bumped wb_thresh may remain high. While > individual devices are under control, the total could exceed > expectations. I agree but this is a potential problem with any kind of bumping based on 'limit - dtc->dirty'. It is just a matter of how many fuse devices you have and how exactly you have max_ratio configured. > Although lowering the max_ratio can avoid this issue, how about reducing > the bumped wb_thresh? > > The formula in my patch: > wb_scale_thresh = (limit - dtc->dirty) / 100; > The intention is to use the default fuse max_ratio(1%) as the multiplier. So basically you propose to use the "/ 8" factor for the normal case and "/ 100" factor for the strictlimit case. My position is that I would not complicate the logic unless somebody comes with a real world setup where the simpler logic is causing real problems. But if you feel strongly about this, I'm fine with that option. Honza -- Jan Kara SUSE Labs, CR