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Thu, 18 Jan 2024 13:40:20 +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 8S2uDkQqqWX9CgAAD6G6ig (envelope-from ); Thu, 18 Jan 2024 13:40:20 +0000 Date: Thu, 18 Jan 2024 14:40:19 +0100 From: Michal Hocko To: Lance Yang Cc: akpm@linux-foundation.org, zokeefe@google.com, david@redhat.com, songmuchun@bytedance.com, shy828301@gmail.com, peterx@redhat.com, mknyszek@google.com, minchan@kernel.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH v2 1/1] mm/madvise: add MADV_F_COLLAPSE_LIGHT to process_madvise() Message-ID: References: <20240118120347.61817-1-ioworker0@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=us-ascii Content-Disposition: inline In-Reply-To: <20240118120347.61817-1-ioworker0@gmail.com> Authentication-Results: smtp-out2.suse.de; none X-Spamd-Result: default: False [0.40 / 50.00]; ARC_NA(0.00)[]; RCVD_VIA_SMTP_AUTH(0.00)[]; FROM_HAS_DN(0.00)[]; TO_DN_SOME(0.00)[]; FREEMAIL_ENVRCPT(0.00)[gmail.com]; TO_MATCH_ENVRCPT_ALL(0.00)[]; MIME_GOOD(-0.10)[text/plain]; RCVD_COUNT_THREE(0.00)[3]; DKIM_SIGNED(0.00)[suse.com:s=susede1]; RCPT_COUNT_SEVEN(0.00)[11]; FREEMAIL_TO(0.00)[gmail.com]; FUZZY_BLOCKED(0.00)[rspamd.com]; FROM_EQ_ENVFROM(0.00)[]; MIME_TRACE(0.00)[0:+]; MID_RHS_NOT_FQDN(0.50)[]; FREEMAIL_CC(0.00)[linux-foundation.org,google.com,redhat.com,bytedance.com,gmail.com,kernel.org,kvack.org,vger.kernel.org]; RCVD_TLS_ALL(0.00)[]; BAYES_HAM(-0.00)[16.10%] X-Spam-Level: X-Spam-Flag: NO X-Spam-Score: 0.40 On Thu 18-01-24 20:03:46, Lance Yang wrote: [...] before we discuss the semantic, let's focus on the usecase. > Use Cases > > An immediate user of this new functionality is the Go runtime heap allocator > that manages memory in hugepage-sized chunks. In the past, whether it was a > newly allocated chunk through mmap() or a reused chunk released by > madvise(MADV_DONTNEED), the allocator attempted to eagerly back memory with > huge pages using madvise(MADV_HUGEPAGE)[2] and madvise(MADV_COLLAPSE)[3] > respectively. However, both approaches resulted in performance issues; for > both scenarios, there could be entries into direct reclaim and/or compaction, > leading to unpredictable stalls[4]. Now, the allocator can confidently use > process_madvise(MADV_F_COLLAPSE_LIGHT) to attempt the allocation of huge pages. IIUC the primary reason is the cost of the huge page allocation which can be really high if the memory is heavily fragmented and it is called synchronously from the process directly, correct? Can that be worked around by process_madvise and performing the operation from a different context? Are there any other reasons to have a different mode? I mean I can think of a more relaxed (opportunistic) MADV_COLLAPSE - e.g. non blocking one to make sure that the caller doesn't really block on resource contention (be it locks or memory availability) because that matches our non-blocking interface in other areas but having a LIGHT operation sounds really vague and the exact semantic would be implementation specific and might change over time. Non-blocking has a clear semantic but it is not really clear whether that is what you really need/want. > [1] https://github.com/torvalds/linux/commit/7d8faaf155454f8798ec56404faca29a82689c77 > [2] https://github.com/golang/go/commit/8fa9e3beee8b0e6baa7333740996181268b60a3a > [3] https://github.com/golang/go/commit/9f9bb26880388c5bead158e9eca3be4b3a9bd2af > [4] https://github.com/golang/go/issues/63334 > > [v1] https://lore.kernel.org/lkml/20240117050217.43610-1-ioworker0@gmail.com/ -- Michal Hocko SUSE Labs