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Date: Mon, 16 Jan 2017 20:44:53 +0800
From: Fam Zheng <famz@redhat.com>
Message-ID: <20170116124453.GA10221@lemon.Home>
References: <20170113131731.1246-1-pbonzini@redhat.com>
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Subject: Re: [Qemu-devel] [PATCH 01/16] aio: introduce aio_co_schedule and
 aio_co_wake
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To: Paolo Bonzini <pbonzini@redhat.com>
Cc: qemu-devel@nongnu.org, stefanha@redhat.com

On Mon, 01/16 13:19, Paolo Bonzini wrote:
> >> +static void co_schedule_bh_cb(void *opaque)
> >> +{
> >> +    AioContext *ctx = opaque;
> >> +    QSLIST_HEAD(, Coroutine) straight, reversed;
> >> +
> >> +    QSLIST_MOVE_ATOMIC(&reversed, &ctx->scheduled_coroutines);
> >> +    QSLIST_INIT(&straight);
> > 
> > Worth special casing 1 element case?
> 
> Sounds like premature optimization; the QSLIST_MOVE_ATOMIC is going to
> be pretty expensive anyway.  Do you mean something like:
> 
> 	if (QSLIST_EMPTY(&reversed)) {
> 	    return;
> 	}
> 	Coroutine *co = QSLIST_FIRST(&reversed);
> 	if (!QSLIST_NEXT(co, co_scheduled_next)) {
> 	    straight = reversed;
> 	} else {
> 	    do {
>                 ...
> 	    } while (!QSLIST_EMPTY(&reversed);
> 	}
> 
> 	do {
> 	    ...
> 	} while (!QSLIST_EMPTY(&straight);
> 
> ?  Looks a but busy.  However, removing the QSLIST_EMPTY case and then
> using do/while may be a nice middle.

I think QSLIST_EMPTY is very unusual. I don't know if these are premature or
not, just asked because the !QSLIST_NEXT() case will be the most common one.

Fam