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From: Marcelo Tosatti <mtosatti@redhat.com>
To: linux-kernel@vger.kernel.org, linux-rt-users@vger.kernel.org
Cc: Thomas Gleixner <tglx@linutronix.de>,
	Anna-Maria Gleixner <anna-maria@linutronix.de>,
	Daniel Bristot de Oliveira <bristot@redhat.com>,
	Luiz Capitulino <lcapitulino@redhat.com>,
	Haris Okanovic <haris.okanovic@ni.com>
Subject: Re: [patch 0/3] do not raise timer softirq unconditionally (spinlockless version)
Date: Mon, 15 Apr 2019 17:17:09 -0300	[thread overview]
Message-ID: <20190415201708.GA14212@amt.cnet> (raw)
In-Reply-To: <20190415201213.600254019@amt.cnet>

On Mon, Apr 15, 2019 at 05:12:13PM -0300, Marcelo Tosatti wrote:
> For isolated CPUs, we'd like to skip awakening ktimersoftd
> (the switch to and then back from ktimersoftd takes 10us in
> virtualized environments, in addition to other OS overhead,
> which exceeds telco requirements for packet forwarding for
> 5G) from the sched tick.
> 
> The patch "timers: do not raise softirq unconditionally" from Thomas
> attempts to address that by checking, in the sched tick, whether its
> necessary to raise the timer softirq. Unfortunately, it attempts to grab
> the tvec base spinlock which generates the issue described in the patch
> "Revert "timers: do not raise softirq unconditionally"".
> 
> tvec_base->lock protects addition of timers to the wheel versus
> timer interrupt execution.
> 
> This patch does not grab the tvec base spinlock from irq context,
> but rather performs a lockless access to base->pending_map.
> 
> It handles the the race between timer addition and timer interrupt
> execution by unconditionally (in case of isolated CPUs) raising the
> timer softirq after making sure the updated bitmap is visible
> on remote CPUs.
> 
> This patchset reduces cyclictest latency from 25us to 14us
> on my testbox. 

Patch against v5.0.5-rt3 branch of linux-rt-devel.


  parent reply	other threads:[~2019-04-15 20:17 UTC|newest]

Thread overview: 18+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-04-15 20:12 [patch 0/3] do not raise timer softirq unconditionally (spinlockless version) Marcelo Tosatti
2019-04-15 20:12 ` [patch 1/3] timers: raise timer softirq on __mod_timer/add_timer_on Marcelo Tosatti
2019-05-29 14:53   ` Anna-Maria Gleixner
2019-05-30 19:23     ` Marcelo Tosatti
2019-04-15 20:12 ` [patch 2/3] timers: do not raise softirq unconditionally (spinlockless version) Marcelo Tosatti
2019-05-29 14:53   ` Anna-Maria Gleixner
2019-05-30 20:14     ` Marcelo Tosatti
2019-05-31 11:55       ` Anna-Maria Gleixner
2019-06-11 11:45         ` Anna-Maria Gleixner
2019-06-04  6:29   ` Peter Xu
2019-06-06 15:14     ` Marcelo Tosatti
2019-04-15 20:12 ` [patch 3/3] timers: condense pending bitmap information Marcelo Tosatti
2019-04-15 20:17 ` Marcelo Tosatti [this message]
2019-05-06  3:22 ` [patch 0/3] do not raise timer softirq unconditionally (spinlockless version) Marcelo Tosatti
2019-05-06  7:17   ` Daniel Bristot de Oliveira
2019-05-06  9:22   ` Thomas Gleixner
2019-05-29 14:52 ` Anna-Maria Gleixner
2019-05-30 19:38   ` Marcelo Tosatti

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