linux-kernel - Re: [PATCH] smp: Allow smp_call_function_single

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Date:   Tue, 17 Dec 2019 15:48:39 -0500
From:   Peter Xu <peterx@...hat.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     linux-kernel@...r.kernel.org,
        Marcelo Tosatti <mtosatti@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Nadav Amit <namit@...are.com>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Subject: Re: [PATCH] smp: Allow smp_call_function_single_async() to insert
 locked csd

On Tue, Dec 17, 2019 at 09:23:52PM +0100, Peter Zijlstra wrote:
> > > I suspect to be nice for virt. Both CPUID and MSR accesses can trap. but
> > > now I'm confused, because it is mostly WRMSR that traps.
> > > 
> > > Anyway, see the commit here: 07cde313b2d2 ("x86/msr: Allow rdmsr_safe_on_cpu() to schedule")
> > 
> > Yes that makes sense.  Thanks for the pointer.
> > 
> > However, then my next confusion is why they can't provide a common
> > solution to the smp code again... I feel like it could be even easier
> > (please see below).  I'm not very familiar with smp code yet, but if
> > it works it should benefit all callers imho.
> 
> Ah, so going to sleep on wait_for_completion() is _much_ more expensive
> than a short spin. So it all depends on the expected behaviour of the
> IPI I suppose.
> 
> In general we expect these IPIs to be 'quick'.
> 
> Also, as is, you're allowed to use the smp_call_function*() family with
> preemption disabled, which pretty much precludes using
> wait_for_completion().

Yes you are right, thanks!

Though I feel like previous small patch could still be an small
enhancement in that it at least shortened the get_cpu() period of
smp_call_function_single() without losing anything - so that could
still give the caller a chance to be scheduled out of the spinning at
some point when the IPI conditionally takes time to finish, imho.

-- 
Peter Xu