From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from mail-pl1-f177.google.com (mail-pl1-f177.google.com [209.85.214.177]) (using TLSv1.2 with cipher ECDHE-RSA-AES128-GCM-SHA256 (128/128 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id D7BB61509B3 for ; Wed, 22 May 2024 23:52:03 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=209.85.214.177 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1716421925; cv=none; b=TLW4iXQy30nvo5fljDGcZOQZjKhuMydYB6gYC5dPs/wgCIzxGmd0mn7olLdB7kg9NVLGE8oqFTYl1CLx6jPG+TZaLQR3apaAyDt3DASvboiWBYl1DWIt8Q2wJ22wFdTkhxddeLu1uE244Klw4lRTOhEfgbxKTajTV+bnMR5xooo= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1716421925; c=relaxed/simple; bh=Pmnm7rhzxz2k9acIiTWsEWfHDNkUBt1qYi5Zk39lXB0=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=Fqo4iW6Js5cBT4F+X6MGV+pJB4c0XiRUcSFgtju5eLlhTP/gre2nfKFX0CAs2tPYYL5rQynzO+RTGehakazqbOPqO6Bqs1QCl4ZYjV6aUZoZG2fLfBRSGZXLy7FhaHvdGIT/wlypwZ+BFQoYZ+MHWp4JvW1OgwkSGFXSbM4NjTM= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=fail (p=none dis=none) header.from=kernel.org; spf=pass smtp.mailfrom=gmail.com; arc=none smtp.client-ip=209.85.214.177 Authentication-Results: smtp.subspace.kernel.org; dmarc=fail (p=none dis=none) header.from=kernel.org Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=gmail.com Received: by mail-pl1-f177.google.com with SMTP id d9443c01a7336-1f335e8d493so2469395ad.1 for ; Wed, 22 May 2024 16:52:03 -0700 (PDT) X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20230601; t=1716421923; x=1717026723; h=in-reply-to:content-transfer-encoding:content-disposition :mime-version:references:message-id:subject:cc:to:from:date :x-gm-message-state:from:to:cc:subject:date:message-id:reply-to; bh=YgJe/VU2CHA4O5aH9yebofep1FFi9VwegOd03XFkMT0=; b=SRXqsvBtmKSI3GYUZOg30ucbeRthCGYeNq1lpPj/RAiesP4eLli26zklDhSVyGJTOJ 5Qz38ClINofMdxNuODa97tiNrtBvt+ciGUu3WwXfIJJBWJG7oG0MDfciuXGSSMQ43alM vNHhhSU9Co6jZeZpnneWd14jbEeQCwqxjDhrTqNfFEXeO7ntd5euGIZisJsk6kVah/JI qibnQEPyUyUwpUPOOYGrNLG4DaBalC5qHUMA+ldioM01ObQphee8PW3DD3zuWsbtqCQU 6Nt+JmlhF6A7ARSsM/rk9olTlNHa0+YWLp9FzcZMXz9ZqtJFJ6+YabLrhzYkqeZsBHZg uOsQ== X-Forwarded-Encrypted: i=1; AJvYcCUcR/QQ6Kbez5X8xC4Piiey6JWhcSxFfkxPGzTZRNLfisyLAz2vObJrUkkBEmPeNaDl1l7x9QI+vVSVfg0P+flVy1sE4pHJhMeaVKsg X-Gm-Message-State: AOJu0Ywo8X6B+/FiEsQ0h2dvFQrrdsBMmaZgpJBWEMWrQvW+DN/uVeuW 7EdDXL2c6wxSuu4aY1QyChETH8SLRnMr4q2pShcfLEQNguxB9EDg X-Google-Smtp-Source: AGHT+IFx9sqketrAstF369fJMuliVnj7CDzB4Ud/h+zw6WNkjxLXPUMX9giQK7mMH1W/C8fVySpfvg== X-Received: by 2002:a17:902:ce8f:b0:1f3:33b:ff18 with SMTP id d9443c01a7336-1f31c964f6bmr39555795ad.11.1716421923052; Wed, 22 May 2024 16:52:03 -0700 (PDT) Received: from snowbird ([136.25.84.117]) by smtp.gmail.com with ESMTPSA id d9443c01a7336-1ef0b9d97e9sm245050305ad.17.2024.05.22.16.52.01 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Wed, 22 May 2024 16:52:02 -0700 (PDT) Date: Wed, 22 May 2024 16:52:00 -0700 From: Dennis Zhou To: Mateusz Guzik Cc: tj@kernel.org, hughd@google.com, akpm@linux-foundation.org, vbabka@suse.cz, linux-kernel@vger.kernel.org, linux-mm@kvack.org Subject: Re: [PATCH v3] percpu_counter: add a cmpxchg-based _add_batch variant Message-ID: References: <20240521233100.358002-1-mjguzik@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: On Wed, May 22, 2024 at 06:59:02AM +0200, Mateusz Guzik wrote: > On Wed, May 22, 2024 at 3:17 AM Dennis Zhou wrote: > > > > Hi Mateusz, > > > > On Wed, May 22, 2024 at 01:31:00AM +0200, Mateusz Guzik wrote: > > > Interrupt disable/enable trips are quite expensive on x86-64 compared to > > > a mere cmpxchg (note: no lock prefix!) and percpu counters are used > > > quite often. > > > > > > With this change I get a bump of 1% ops/s for negative path lookups, > > > plugged into will-it-scale: > > > > > > void testcase(unsigned long long *iterations, unsigned long nr) > > > { > > > while (1) { > > > int fd = open("/tmp/nonexistent", O_RDONLY); > > > assert(fd == -1); > > > > > > (*iterations)++; > > > } > > > } > > > > > > The win would be higher if it was not for other slowdowns, but one has > > > to start somewhere. > > > > This is cool! > > > > > > > > Signed-off-by: Mateusz Guzik > > > Acked-by: Vlastimil Babka > > > --- > > > > > > v3: > > > - add a missing word to the new comment > > > > > > v2: > > > - dodge preemption > > > - use this_cpu_try_cmpxchg > > > - keep the old variant depending on CONFIG_HAVE_CMPXCHG_LOCAL > > > > > > lib/percpu_counter.c | 44 +++++++++++++++++++++++++++++++++++++++----- > > > 1 file changed, 39 insertions(+), 5 deletions(-) > > > > > > diff --git a/lib/percpu_counter.c b/lib/percpu_counter.c > > > index 44dd133594d4..c3140276bb36 100644 > > > --- a/lib/percpu_counter.c > > > +++ b/lib/percpu_counter.c > > > @@ -73,17 +73,50 @@ void percpu_counter_set(struct percpu_counter *fbc, s64 amount) > > > EXPORT_SYMBOL(percpu_counter_set); > > > > > > /* > > > - * local_irq_save() is needed to make the function irq safe: > > > - * - The slow path would be ok as protected by an irq-safe spinlock. > > > - * - this_cpu_add would be ok as it is irq-safe by definition. > > > - * But: > > > - * The decision slow path/fast path and the actual update must be atomic, too. > > > + * Add to a counter while respecting batch size. > > > + * > > > + * There are 2 implementations, both dealing with the following problem: > > > + * > > > + * The decision slow path/fast path and the actual update must be atomic. > > > * Otherwise a call in process context could check the current values and > > > * decide that the fast path can be used. If now an interrupt occurs before > > > * the this_cpu_add(), and the interrupt updates this_cpu(*fbc->counters), > > > * then the this_cpu_add() that is executed after the interrupt has completed > > > * can produce values larger than "batch" or even overflows. > > > */ > > > +#ifdef CONFIG_HAVE_CMPXCHG_LOCAL > > > +/* > > > + * Safety against interrupts is achieved in 2 ways: > > > + * 1. the fast path uses local cmpxchg (note: no lock prefix) > > > + * 2. the slow path operates with interrupts disabled > > > + */ > > > +void percpu_counter_add_batch(struct percpu_counter *fbc, s64 amount, s32 batch) > > > +{ > > > + s64 count; > > > + unsigned long flags; > > > + > > > + count = this_cpu_read(*fbc->counters); > > > > Should this_cpu_read() be inside the do {} while in case the extreme > > case that we get preempted after the read and before the cmpxchg AND > > count + amount < batch on both the previous and next cpu? > > > > this_cpu_try_cmpxchg updates the local value on failure (hence &), so > from semantic pov this is equivalent to having this_cpu_read in the > loop. I'm using it the same way as mod_zone_state. > Ah I didn't catch that last night. Thanks. I've applied this to percpu#for-6.11. Thanks, Dennis > > > + do { > > > + if (unlikely(abs(count + amount)) >= batch) { > > > + raw_spin_lock_irqsave(&fbc->lock, flags); > > > + /* > > > + * Note: by now we might have migrated to another CPU > > > + * or the value might have changed. > > > + */ > > > + count = __this_cpu_read(*fbc->counters); > > > + fbc->count += count + amount; > > > + __this_cpu_sub(*fbc->counters, count); > > > + raw_spin_unlock_irqrestore(&fbc->lock, flags); > > > + return; > > > + } > > > + } while (!this_cpu_try_cmpxchg(*fbc->counters, &count, count + amount)); > > > +} > > > +#else > > > +/* > > > + * local_irq_save() is used to make the function irq safe: > > > + * - The slow path would be ok as protected by an irq-safe spinlock. > > > + * - this_cpu_add would be ok as it is irq-safe by definition. > > > + */ > > > void percpu_counter_add_batch(struct percpu_counter *fbc, s64 amount, s32 batch) > > > { > > > s64 count; > > > @@ -101,6 +134,7 @@ void percpu_counter_add_batch(struct percpu_counter *fbc, s64 amount, s32 batch) > > > } > > > local_irq_restore(flags); > > > } > > > +#endif > > > EXPORT_SYMBOL(percpu_counter_add_batch); > > > > > > /* > > > -- > > > 2.39.2 > > > > > > > Thanks, > > Dennis > > > > -- > Mateusz Guzik