From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from desiato.infradead.org (desiato.infradead.org [90.155.92.199]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id D28B030BF7B for ; Thu, 6 Nov 2025 09:30:04 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=90.155.92.199 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1762421411; cv=none; b=ux8EpWoSo8NMH1D2W3uIvZ3hgOrl7syIHsn78DGY0N0/ygkWDrJgIgSSJgAfqXNTEnEcNhU6hG0B5jd17//WpfmQK9vB0IprN5Viw8RZQvzDiCUJjeLodm+3RhMUquUYNfC3sT+w4++UEL8/veRwe108FRJEAgwfU7M6aAyJYOQ= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1762421411; c=relaxed/simple; bh=5Q70uNZgvXMnG+FcWTm2thBI5YWNuANk3UsX8UcykQo=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=tQRhn/wPi8ExBw8bZ5X5e0i5X/XBwwhFGQyLDFYW1ynqS4y0BMSuHnbdE8CQXUUmeRYroVUilM5RDEDfxWkZAwpevvi+cmrVY+ANTXEjtw5Ch1+vyA+cZ/AC/p6n3kJ0CQQ9131thfpxXNtkXyVKEhPO1KUveaxdHU19h+FXUOg= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=infradead.org; spf=none smtp.mailfrom=infradead.org; dkim=pass (2048-bit key) header.d=infradead.org header.i=@infradead.org header.b=LHD90CAO; arc=none smtp.client-ip=90.155.92.199 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=infradead.org Authentication-Results: smtp.subspace.kernel.org; spf=none smtp.mailfrom=infradead.org Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=infradead.org header.i=@infradead.org header.b="LHD90CAO" DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=infradead.org; s=desiato.20200630; h=In-Reply-To:Content-Type:MIME-Version: References:Message-ID:Subject:Cc:To:From:Date:Sender:Reply-To: Content-Transfer-Encoding:Content-ID:Content-Description; bh=Z4MczQLnGpbhKtlh5ALVW+kByWj5MWmj7cgNf4sxqp8=; b=LHD90CAO3edTfbPh6pxHU4cTUu EinH0K2GaUMU/XKVWaysK3lnoiJ9l5hZ4urWkL6YwcUI6rauumxRaqXkD4LzA6pv2nbD8r5yNdPgH 7MVgG+CZ40f0XTQakHCI2tmgOH2rMSlD2nOQshhNgK+bfhcHinFbF3sqPIbplQJyN8ai2ltYv8qBz waxqx+Snwltd/LmfS4yjSF0Bs4DnIPjeiyBdLacGJD3OozwArpOm+Jw0lC1O6qfmNo+PoEtRA71oN /eT77mBoQpw0zq6kGO3LXWVmS5foFse2la1Wq+qgfEgGdnHkxmn7x/p7wSggZs8ciuaCOQA9TeNFF Pj8y1KEg==; Received: from 77-249-17-252.cable.dynamic.v4.ziggo.nl ([77.249.17.252] helo=noisy.programming.kicks-ass.net) by desiato.infradead.org with esmtpsa (Exim 4.98.2 #2 (Red Hat Linux)) id 1vGvRS-00000003sQs-00NQ; Thu, 06 Nov 2025 08:34:19 +0000 Received: by noisy.programming.kicks-ass.net (Postfix, from userid 1000) id 66B6F300265; Thu, 06 Nov 2025 10:29:29 +0100 (CET) Date: Thu, 6 Nov 2025 10:29:29 +0100 From: Peter Zijlstra To: Shrikanth Hegde Cc: Sebastian Andrzej Siewior , =?iso-8859-1?Q?Andr=E9?= Almeida , Darren Hart , Davidlohr Bueso , Ingo Molnar , Juri Lelli , Thomas Gleixner , linux-kernel@vger.kernel.org, Valentin Schneider , Waiman Long Subject: Re: [PATCH v2 0/6] futex: Use RCU-based per-CPU reference counting Message-ID: <20251106092929.GR4067720@noisy.programming.kicks-ass.net> References: <20250710110011.384614-1-bigeasy@linutronix.de> <20250715163134.pM1J2XO9@linutronix.de> <88edcfdf-2253-4563-a895-6e8bb1625800@linux.ibm.com> <20250716142946.GD905792@noisy.programming.kicks-ass.net> 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: On Wed, Jul 16, 2025 at 11:51:46PM +0530, Shrikanth Hegde wrote: > > Anyway, I think we can improve both. Does the below help? > > > > > > --- > > diff --git a/kernel/futex/core.c b/kernel/futex/core.c > > index d9bb5567af0c..8c41d050bd1f 100644 > > --- a/kernel/futex/core.c > > +++ b/kernel/futex/core.c > > @@ -1680,10 +1680,10 @@ static bool futex_ref_get(struct futex_private_hash *fph) > > { > > struct mm_struct *mm = fph->mm; > > - guard(rcu)(); > > + guard(preempt)(); > > - if (smp_load_acquire(&fph->state) == FR_PERCPU) { > > - this_cpu_inc(*mm->futex_ref); > > + if (READ_ONCE(fph->state) == FR_PERCPU) { > > + __this_cpu_inc(*mm->futex_ref); > > return true; > > } > > @@ -1694,10 +1694,10 @@ static bool futex_ref_put(struct futex_private_hash *fph) > > { > > struct mm_struct *mm = fph->mm; > > - guard(rcu)(); > > + guard(preempt)(); > > - if (smp_load_acquire(&fph->state) == FR_PERCPU) { > > - this_cpu_dec(*mm->futex_ref); > > + if (READ_ONCE(fph->state) == FR_PERCPU) { > > + __this_cpu_dec(*mm->futex_ref); > > return false; > > } > > Yes. It helps. It improves "-b 512" numbers by at-least 5%. While talking with Sebastian about this work, I realized this patch was never committed. So I've written it up like so, and will commit to tip/locking/urgent soonish. --- Subject: futex: Optimize per-cpu reference counting From: Peter Zijlstra Date: Wed, 16 Jul 2025 16:29:46 +0200 Shrikanth noted that the per-cpu reference counter was still some 10% slower than the old immutable option (which removes the reference counting entirely). Further optimize the per-cpu reference counter by: - switching from RCU to preempt; - using __this_cpu_*() since we now have preempt disabled; - switching from smp_load_acquire() to READ_ONCE(). This is all safe because disabling preemption inhibits the RCU grace period exactly like rcu_read_lock(). Having preemption disabled allows using __this_cpu_*() provided the only access to the variable is in task context -- which is the case here. Furthermore, since we know changing fph->state to FR_ATOMIC demands a full RCU grace period we can rely on the implied smp_mb() from that to replace the acquire barrier(). This is very similar to the percpu_down_read_internal() fast-path. The reason this is significant for PowerPC is that it uses the generic this_cpu_*() implementation which relies on local_irq_disable() (the x86 implementation relies on it being a single memop instruction to be IRQ-safe). Switching to preempt_disable() and __this_cpu*() avoids this IRQ state swizzling. Also, PowerPC needs LWSYNC for the ACQUIRE barrier, not having to use explicit barriers safes a bunch. Combined this reduces the performance gap by half, down to some 5%. Reported-by: Shrikanth Hegde Tested-by: Shrikanth Hegde Signed-off-by: Peter Zijlstra (Intel) --- kernel/futex/core.c | 12 ++++++------ 1 file changed, 6 insertions(+), 6 deletions(-) --- a/kernel/futex/core.c +++ b/kernel/futex/core.c @@ -1680,10 +1680,10 @@ static bool futex_ref_get(struct futex_p { struct mm_struct *mm = fph->mm; - guard(rcu)(); + guard(preempt)(); - if (smp_load_acquire(&fph->state) == FR_PERCPU) { - this_cpu_inc(*mm->futex_ref); + if (READ_ONCE(fph->state) == FR_PERCPU) { + __this_cpu_inc(*mm->futex_ref); return true; } @@ -1694,10 +1694,10 @@ static bool futex_ref_put(struct futex_p { struct mm_struct *mm = fph->mm; - guard(rcu)(); + guard(preempt)(); - if (smp_load_acquire(&fph->state) == FR_PERCPU) { - this_cpu_dec(*mm->futex_ref); + if (READ_ONCE(fph->state) == FR_PERCPU) { + __this_cpu_dec(*mm->futex_ref); return false; }