netdev - Re: [PATCH] net: sched: dev_deactivate_many(): use msleep(1) instead of yield() to wait for outstanding qdisc

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Message-ID: <1396437459.3989.39.camel@edumazet-glaptop2.roam.corp.google.com>
Date:	Wed, 02 Apr 2014 04:17:39 -0700
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Thomas Gleixner <tglx@...utronix.de>,
	David Miller <davem@...emloft.net>, ben@...adent.org.uk,
	mkl@...gutronix.de,
	linux-rt-users <linux-rt-users@...r.kernel.org>,
	LKML <linux-kernel@...r.kernel.org>, netdev@...r.kernel.org,
	kernel@...gutronix.de
Subject: Re: [PATCH] net: sched: dev_deactivate_many(): use msleep(1)
 instead of yield() to wait for outstanding qdisc_run callsb

On Wed, 2014-04-02 at 13:07 +0200, Peter Zijlstra wrote:
> On Mon, Mar 31, 2014 at 11:49:16PM +0200, Thomas Gleixner wrote:
> > > >> The intent of a yield() call, like this one here, is unambiguously
> > > >> that the current thread cannot do anything until some other thread
> > > >> gets onto the cpu and makes forward progress.
> 
> Yeah; like Thomas said; that's not what yield() does -- or can do.
> 
> yield() only says 'I don't know what to do, you figure it out', but then
> fails to provide enough information to actually do something sensible.
> 
> It cannot put the task to sleep; because it doesn't pair with a wakeup;
> and therefore the task stays an eligible/runnable task from a scheduler
> pov.
> 
> The scheduler is therefore entirely in its right to pick this task
> again; and pretty much has to under many circumstances.
> 
> yield() does not, and can not, guarantee forward progress - ever.
> 
> Use wait_event() and assorted bits to wait for an actual event. That is
> a sleep paired with a wakeup and thus has progress guarantees.

Why wouldn't it be safe to redefine yield as msleep(1) ?

net/ipv4/tcp_output.c seems to also use yield().


                /* Socket is locked, keep trying until memory is available. */
                for (;;) {
                        skb = alloc_skb_fclone(MAX_TCP_HEADER,
                                               sk->sk_allocation);
                        if (skb)
                                break;
                        yield();
                }


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