linux-kernel - Re: [patch 0/3] do not raise timer softirq unconditionally (spinlockless version)

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Message-ID: <20190506032234.GA31395@amt.cnet>
Date:   Mon, 6 May 2019 00:22:34 -0300
From:   Marcelo Tosatti <mtosatti@...hat.com>
To:     linux-kernel@...r.kernel.org, linux-rt-users@...r.kernel.org
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Anna-Maria Gleixner <anna-maria@...utronix.de>,
        Daniel Bristot de Oliveira <bristot@...hat.com>,
        Luiz Capitulino <lcapitulino@...hat.com>,
        Haris Okanovic <haris.okanovic@...com>
Subject: Re: [patch 0/3] do not raise timer softirq unconditionally
 (spinlockless version)

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. 
> 
> 

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