From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id 1D974CDB483 for ; Wed, 18 Oct 2023 09:52:35 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S229774AbjJRJwe (ORCPT ); Wed, 18 Oct 2023 05:52:34 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:45814 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229482AbjJRJwc (ORCPT ); Wed, 18 Oct 2023 05:52:32 -0400 Received: from smtp-out1.suse.de (smtp-out1.suse.de [195.135.220.28]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 68F25B0; Wed, 18 Oct 2023 02:52:30 -0700 (PDT) Received: from imap2.suse-dmz.suse.de (imap2.suse-dmz.suse.de [192.168.254.74]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature ECDSA (P-521) server-digest SHA512) (No client certificate requested) by smtp-out1.suse.de (Postfix) with ESMTPS id 7611221855; Wed, 18 Oct 2023 09:52:28 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=suse.cz; s=susede2_rsa; t=1697622748; h=from:from:reply-to:date:date:message-id:message-id:to:to:cc:cc: mime-version:mime-version:content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=MP/5D96ZpJ3W3396Dnkowli5D0mm+jI2fuulfFVaRH4=; b=HWdnzJipDJW47U8wwqs1mO8VBtEZtLc2FZuVAvu5q3/M6Xrf/fZiffvPfVNezbOD61rzB0 WPV5KXVtanD8ggDDL1GqeL3D+8dOa4idhgQROPeguBYugRGJ3cU/VfiBv3SqwN7f/hPLv0 PmITPPW5VcJhEOcLN5a8CGb9clGIo4Q= DKIM-Signature: v=1; a=ed25519-sha256; c=relaxed/relaxed; d=suse.cz; s=susede2_ed25519; t=1697622748; h=from:from:reply-to:date:date:message-id:message-id:to:to:cc:cc: mime-version:mime-version:content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=MP/5D96ZpJ3W3396Dnkowli5D0mm+jI2fuulfFVaRH4=; b=jO5ls5UPDrSlQqt/aZlxP9bj0hL9rzQ5gpUAOJuumBwpibl2zQfZxkskTonAAmco5uHeFo uL4h5WTgkS3uA9BQ== Received: from imap2.suse-dmz.suse.de (imap2.suse-dmz.suse.de [192.168.254.74]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature ECDSA (P-521) server-digest SHA512) (No client certificate requested) by imap2.suse-dmz.suse.de (Postfix) with ESMTPS id 482E913915; Wed, 18 Oct 2023 09:52:28 +0000 (UTC) Received: from dovecot-director2.suse.de ([192.168.254.65]) by imap2.suse-dmz.suse.de with ESMTPSA id VxjJENyqL2W/OQAAMHmgww (envelope-from ); Wed, 18 Oct 2023 09:52:28 +0000 Message-ID: Date: Wed, 18 Oct 2023 11:52:27 +0200 MIME-Version: 1.0 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:102.0) Gecko/20100101 Thunderbird/102.15.1 Subject: Re: [PATCH v3 2/5] mm: kmem: add direct objcg pointer to task_struct To: Roman Gushchin , Andrew Morton Cc: linux-kernel@vger.kernel.org, cgroups@vger.kernel.org, Johannes Weiner , Michal Hocko , shakeelb@google.com, Muchun Song , Dennis Zhou , David Rientjes , Naresh Kamboju References: <20231016221900.4031141-1-roman.gushchin@linux.dev> <20231016221900.4031141-3-roman.gushchin@linux.dev> Content-Language: en-US From: Vlastimil Babka In-Reply-To: <20231016221900.4031141-3-roman.gushchin@linux.dev> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit Authentication-Results: smtp-out1.suse.de; none X-Spamd-Result: default: False [-7.10 / 50.00]; ARC_NA(0.00)[]; RCVD_VIA_SMTP_AUTH(0.00)[]; BAYES_HAM(-3.00)[100.00%]; FROM_HAS_DN(0.00)[]; TO_DN_SOME(0.00)[]; TO_MATCH_ENVRCPT_ALL(0.00)[]; NEURAL_HAM_LONG(-3.00)[-1.000]; MIME_GOOD(-0.10)[text/plain]; DKIM_SIGNED(0.00)[suse.cz:s=susede2_rsa,suse.cz:s=susede2_ed25519]; NEURAL_HAM_SHORT(-1.00)[-1.000]; RCPT_COUNT_SEVEN(0.00)[11]; FROM_EQ_ENVFROM(0.00)[]; MIME_TRACE(0.00)[0:+]; RCVD_COUNT_TWO(0.00)[2]; RCVD_TLS_ALL(0.00)[]; MID_RHS_MATCH_FROM(0.00)[] Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 10/17/23 00:18, Roman Gushchin wrote: > To charge a freshly allocated kernel object to a memory cgroup, the > kernel needs to obtain an objcg pointer. Currently it does it > indirectly by obtaining the memcg pointer first and then calling to > __get_obj_cgroup_from_memcg(). > > Usually tasks spend their entire life belonging to the same object > cgroup. So it makes sense to save the objcg pointer on task_struct > directly, so it can be obtained faster. It requires some work on fork, > exit and cgroup migrate paths, but these paths are way colder. > > To avoid any costly synchronization the following rules are applied: > 1) A task sets it's objcg pointer itself. > > 2) If a task is being migrated to another cgroup, the least > significant bit of the objcg pointer is set atomically. > > 3) On the allocation path the objcg pointer is obtained locklessly > using the READ_ONCE() macro and the least significant bit is > checked. If it's set, the following procedure is used to update > it locklessly: > - task->objcg is zeroed using cmpxcg > - new objcg pointer is obtained > - task->objcg is updated using try_cmpxchg > - operation is repeated if try_cmpxcg fails > It guarantees that no updates will be lost if task migration > is racing against objcg pointer update. It also allows to keep > both read and write paths fully lockless. > > Because the task is keeping a reference to the objcg, it can't go away > while the task is alive. > > This commit doesn't change the way the remote memcg charging works. > > Signed-off-by: Roman Gushchin (Cruise) > Tested-by: Naresh Kamboju > Acked-by: Johannes Weiner > --- > include/linux/sched.h | 4 ++ > mm/memcontrol.c | 130 +++++++++++++++++++++++++++++++++++++++--- > 2 files changed, 125 insertions(+), 9 deletions(-) > > diff --git a/include/linux/sched.h b/include/linux/sched.h > index 77f01ac385f7..60de42715b56 100644 > --- a/include/linux/sched.h > +++ b/include/linux/sched.h > @@ -1443,6 +1443,10 @@ struct task_struct { > struct mem_cgroup *active_memcg; > #endif > > +#ifdef CONFIG_MEMCG_KMEM > + struct obj_cgroup *objcg; > +#endif > + > #ifdef CONFIG_BLK_CGROUP > struct gendisk *throttle_disk; > #endif > diff --git a/mm/memcontrol.c b/mm/memcontrol.c > index 16ac2a5838fb..0605e45bd4a2 100644 > --- a/mm/memcontrol.c > +++ b/mm/memcontrol.c > @@ -249,6 +249,8 @@ struct mem_cgroup *vmpressure_to_memcg(struct vmpressure *vmpr) > return container_of(vmpr, struct mem_cgroup, vmpressure); > } > > +#define CURRENT_OBJCG_UPDATE_FLAG 0x1UL There's a silent relation between this and set_bit(0, ...) in mem_cgroup_kmem_attach(), maybe worth a comment at least, or defining the bit number first and from that the flag? > + > #ifdef CONFIG_MEMCG_KMEM > static DEFINE_SPINLOCK(objcg_lock); > > @@ -3001,6 +3003,50 @@ static struct obj_cgroup *__get_obj_cgroup_from_memcg(struct mem_cgroup *memcg) > return objcg; > } > > +static struct obj_cgroup *current_objcg_update(void) > +{ > + struct mem_cgroup *memcg; > + struct obj_cgroup *old, *objcg = NULL; > + > + do { > + /* Atomically drop the update bit. */ > + old = xchg(¤t->objcg, NULL); > + if (old) { > + old = (struct obj_cgroup *) > + ((unsigned long)old & ~CURRENT_OBJCG_UPDATE_FLAG); > + if (old) > + obj_cgroup_put(old); > + > + old = NULL; > + } > + > + /* Obtain the new objcg pointer. */ > + rcu_read_lock(); > + memcg = mem_cgroup_from_task(current); > + /* > + * The current task can be asynchronously moved to another > + * memcg and the previous memcg can be offlined. So let's > + * get the memcg pointer and try get a reference to objcg > + * under a rcu read lock. > + */ > + for (; memcg != root_mem_cgroup; memcg = parent_mem_cgroup(memcg)) { > + objcg = rcu_dereference(memcg->objcg); > + if (likely(objcg && obj_cgroup_tryget(objcg))) So IIUC here we increase objcg refcount. > + break; > + objcg = NULL; > + } > + rcu_read_unlock(); > + > + /* > + * Try set up a new objcg pointer atomically. If it > + * fails, it means the update flag was set concurrently, so > + * the whole procedure should be repeated. > + */ > + } while (!try_cmpxchg(¤t->objcg, &old, objcg)); And if this fails we throw objcg away and try again, but we should do obj_cgroup_put(objcg) first, as otherwise it would cause a leak? > + > + return objcg; > +} > + > __always_inline struct obj_cgroup *get_obj_cgroup_from_current(void) > { > struct mem_cgroup *memcg; > @@ -3008,19 +3054,26 @@ __always_inline struct obj_cgroup *get_obj_cgroup_from_current(void) > > if (in_task()) { > memcg = current->active_memcg; > + if (unlikely(memcg)) > + goto from_memcg; > > - /* Memcg to charge can't be determined. */ > - if (likely(!memcg) && (!current->mm || (current->flags & PF_KTHREAD))) The checks for current->mm and PF_KTHREAD seem to be gone completely after the patch, was that intended and why? > - return NULL; > + objcg = READ_ONCE(current->objcg); > + if (unlikely((unsigned long)objcg & CURRENT_OBJCG_UPDATE_FLAG)) > + objcg = current_objcg_update(); > + > + if (objcg) { > + obj_cgroup_get(objcg); > + return objcg; > + } > } else { > memcg = this_cpu_read(int_active_memcg); > - if (likely(!memcg)) > - return NULL; > + if (unlikely(memcg)) > + goto from_memcg; > } > + return NULL; > > +from_memcg: > rcu_read_lock(); > - if (!memcg) > - memcg = mem_cgroup_from_task(current); > objcg = __get_obj_cgroup_from_memcg(memcg); > rcu_read_unlock(); > return objcg; > @@ -6345,6 +6398,7 @@ static void mem_cgroup_move_task(void) > mem_cgroup_clear_mc(); > } > } > +