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Subject: Re: [PATCH 1/8] scsi: Drop struct Scsi_Host->host_lock around
 SHT->queuecommand()
From: James Bottomley <James.Bottomley@suse.de>
To: Andi Kleen <ak@linux.intel.com>
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Date: Fri, 17 Sep 2010 10:57:18 -0400
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On Fri, 2010-09-17 at 16:22 +0200, Andi Kleen wrote:
> > I don't disagree with the idea of removing it, especially as it has so
> > few users, but replacing the host lock with an atomic here would still
> > vastly reduce the contention, which is the initial complaint.  The
> 
> Actually the complaint is the overhead of the spin lock. This can be 
> either caused
> by contention or by cache line bounce time.

The original complaint was contention.  My desire is to reduce the
locked path coverage, so I saw an opportunity.

What I was actually thinking of for the atomic is that we'd let it range
[1..INT_MAX] so a zero was an indicator of no use of this.  Then the
actual code could become

if (atomic_read(x)) {
	do {
		y = atomic_add_return(1, x);
	} while (y == 0);
}

So "fast" cards not using the serial number set a zero there (we'd
default initialise to one), the line is shared so no bouncing (because
it's never updated).  This should satisfy everyone.

> > contention occurs because the host lock is so widely used for other
> > things.  The way to reduce that contention is firstly to reduce the
> > length of code covered by the lock and also reduce the actual number of
> > places where the lock is taken.  Compared with host lock's current vast
> > footprint, and atomic here is tiny.
> 
> That assumes that it's contention that is the problem and not simply 
> bounce time.

That's what the patch and data that started this whole thread showed,
yes ... but I think actual bounce in the spinlock is also a problem ...
we just don't have data to show it.

James