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Wed, 10 Sep 2025 08:01:06 +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 mPqkCUIwwWgGJAAAD6G6ig (envelope-from ); Wed, 10 Sep 2025 08:01:06 +0000 From: Vlastimil Babka Date: Wed, 10 Sep 2025 10:01:08 +0200 Subject: [PATCH v8 06/23] slab: determine barn status racily outside of lock 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-Transfer-Encoding: quoted-printable Message-Id: <20250910-slub-percpu-caches-v8-6-ca3099d8352c@suse.cz> References: <20250910-slub-percpu-caches-v8-0-ca3099d8352c@suse.cz> In-Reply-To: <20250910-slub-percpu-caches-v8-0-ca3099d8352c@suse.cz> To: Suren Baghdasaryan , "Liam R. Howlett" , Christoph Lameter , David Rientjes Cc: Roman Gushchin , Harry Yoo , Uladzislau Rezki , Sidhartha Kumar , linux-mm@kvack.org, linux-kernel@vger.kernel.org, rcu@vger.kernel.org, maple-tree@lists.infradead.org, vbabka@suse.cz X-Mailer: b4 0.14.2 X-Spamd-Result: default: False [-4.30 / 50.00]; BAYES_HAM(-3.00)[100.00%]; NEURAL_HAM_LONG(-1.00)[-1.000]; NEURAL_HAM_SHORT(-0.20)[-0.999]; MIME_GOOD(-0.10)[text/plain]; RCVD_VIA_SMTP_AUTH(0.00)[]; ARC_NA(0.00)[]; FREEMAIL_ENVRCPT(0.00)[gmail.com]; MIME_TRACE(0.00)[0:+]; FUZZY_RATELIMITED(0.00)[rspamd.com]; TO_DN_SOME(0.00)[]; RCPT_COUNT_TWELVE(0.00)[13]; RCVD_TLS_ALL(0.00)[]; MID_RHS_MATCH_FROM(0.00)[]; DKIM_SIGNED(0.00)[suse.cz:s=susede2_rsa,suse.cz:s=susede2_ed25519]; FROM_HAS_DN(0.00)[]; FREEMAIL_CC(0.00)[linux.dev,oracle.com,gmail.com,kvack.org,vger.kernel.org,lists.infradead.org,suse.cz]; R_RATELIMIT(0.00)[to_ip_from(RLwn5r54y1cp81no5tmbbew5oc)]; FROM_EQ_ENVFROM(0.00)[]; TO_MATCH_ENVRCPT_ALL(0.00)[]; RCVD_COUNT_TWO(0.00)[2]; DBL_BLOCKED_OPENRESOLVER(0.00)[suse.cz:mid,suse.cz:email] X-Spam-Flag: NO X-Spam-Level: X-Spam-Score: -4.30 The possibility of many barn operations is determined by the current number of full or empty sheaves. Taking the barn->lock just to find out that e.g. there are no empty sheaves results in unnecessary overhead and lock contention. Thus perform these checks outside of the lock with a data_race() annotated variable read and fail quickly without taking the lock. Checks for sheaf availability that racily succeed have to be obviously repeated under the lock for correctness, but we can skip repeating checks if there are too many sheaves on the given list as the limits don't need to be strict. Reviewed-by: Suren Baghdasaryan Reviewed-by: Harry Yoo Signed-off-by: Vlastimil Babka --- mm/slub.c | 27 ++++++++++++++++++++------- 1 file changed, 20 insertions(+), 7 deletions(-) diff --git a/mm/slub.c b/mm/slub.c index 38f5b865d3093556171e0f6530d395718b438099..35274ce4e709c9da7ac8f9006c8= 24f28709e923d 100644 --- a/mm/slub.c +++ b/mm/slub.c @@ -2801,9 +2801,12 @@ static struct slab_sheaf *barn_get_empty_sheaf(struc= t node_barn *barn) struct slab_sheaf *empty =3D NULL; unsigned long flags; =20 + if (!data_race(barn->nr_empty)) + return NULL; + spin_lock_irqsave(&barn->lock, flags); =20 - if (barn->nr_empty) { + if (likely(barn->nr_empty)) { empty =3D list_first_entry(&barn->sheaves_empty, struct slab_sheaf, barn_list); list_del(&empty->barn_list); @@ -2850,6 +2853,9 @@ static struct slab_sheaf *barn_get_full_or_empty_shea= f(struct node_barn *barn) struct slab_sheaf *sheaf =3D NULL; unsigned long flags; =20 + if (!data_race(barn->nr_full) && !data_race(barn->nr_empty)) + return NULL; + spin_lock_irqsave(&barn->lock, flags); =20 if (barn->nr_full) { @@ -2880,9 +2886,12 @@ barn_replace_empty_sheaf(struct node_barn *barn, str= uct slab_sheaf *empty) struct slab_sheaf *full =3D NULL; unsigned long flags; =20 + if (!data_race(barn->nr_full)) + return NULL; + spin_lock_irqsave(&barn->lock, flags); =20 - if (barn->nr_full) { + if (likely(barn->nr_full)) { full =3D list_first_entry(&barn->sheaves_full, struct slab_sheaf, barn_list); list_del(&full->barn_list); @@ -2906,19 +2915,23 @@ barn_replace_full_sheaf(struct node_barn *barn, str= uct slab_sheaf *full) struct slab_sheaf *empty; unsigned long flags; =20 + /* we don't repeat this check under barn->lock as it's not critical */ + if (data_race(barn->nr_full) >=3D MAX_FULL_SHEAVES) + return ERR_PTR(-E2BIG); + if (!data_race(barn->nr_empty)) + return ERR_PTR(-ENOMEM); + spin_lock_irqsave(&barn->lock, flags); =20 - if (barn->nr_full >=3D MAX_FULL_SHEAVES) { - empty =3D ERR_PTR(-E2BIG); - } else if (!barn->nr_empty) { - empty =3D ERR_PTR(-ENOMEM); - } else { + if (likely(barn->nr_empty)) { empty =3D list_first_entry(&barn->sheaves_empty, struct slab_sheaf, barn_list); list_del(&empty->barn_list); list_add(&full->barn_list, &barn->sheaves_full); barn->nr_empty--; barn->nr_full++; + } else { + empty =3D ERR_PTR(-ENOMEM); } =20 spin_unlock_irqrestore(&barn->lock, flags); --=20 2.51.0