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Date: Wed, 01 Sep 2010 12:04:51 -0500
From: Alan Cox <alc@rice.edu>
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X-Mailman-Approved-At: Wed, 01 Sep 2010 17:24:48 +0000
Cc: freebsd-current@freebsd.org, Jeff Roberson <jroberson@jroberson.net>
Subject: Re: sched_pin() bug in SCHED_ULE
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mdf@FreeBSD.org wrote:
[snip]
> I will test this patch out; thanks for the help!
>
> Two questions:
>
> 1) How does a thread get moved between CPUs when it's not running?  I
> see that we change the runqueue for non-running threads that are on a
> runqueue.  Does the code always check for THREAD_CAN_SCHED when adding
> a sleeping thread to a runqueue on wakeup?
>
> 2) I assume sched_switch() runs a lot more often than sched_pin() or
> sched_affinity(), so it would make sense to add any performance
> penalty of increased work in either of those places than in the
> scheduler.  I suppose the two memory references for THREAD_CAN_MIGRATE
> and THREAD_CAN_SCHED won't be that expensive.
>   

sched_pin() gets used a fair amount on i386 for creating temporary 
mappings in order to avoid the need for system-wide TLB shootdowns.  The 
use cases range from the fast path for pipes to page zeroing.

Alan