linux-kernel - Re: [PATCH 1/8] scsi: Drop struct Scsi_Host->host

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Date:	Fri, 17 Sep 2010 09:41:44 -0700
From:	"Nicholas A. Bellinger" <nab@...ux-iscsi.org>
To:	James Bottomley <James.Bottomley@...e.de>
Cc:	Andi Kleen <ak@...ux.intel.com>,
	linux-scsi <linux-scsi@...r.kernel.org>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	Vasu Dev <vasu.dev@...ux.intel.com>,
	Tim Chen <tim.c.chen@...ux.intel.com>,
	Matthew Wilcox <willy@...ux.intel.com>,
	Mike Christie <michaelc@...wisc.edu>,
	James Smart <james.smart@...lex.com>,
	Andrew Vasquez <andrew.vasquez@...gic.com>,
	FUJITA Tomonori <fujita.tomonori@....ntt.co.jp>,
	Hannes Reinecke <hare@...e.de>,
	Joe Eykholt <jeykholt@...co.com>,
	Christoph Hellwig <hch@....de>
Subject: Re: [PATCH 1/8] scsi: Drop struct Scsi_Host->host_lock around
	SHT->queuecommand()

On Fri, 2010-09-17 at 09:37 -0700, Nicholas A. Bellinger wrote:
> On Fri, 2010-09-17 at 10:57 -0400, James Bottomley wrote:
> > 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);
> > }
> 
> The conversion of struct scsi_cmnd->serial_number to atomic_t and the
> above code for scsi_cmd_get_serial() sounds perfectly reasonable to me.
> 

Actually, that should be the conversion of struct
Scsi_Host->cmd_serial_number to an atomic_t.   AFAICT there is no reason
for struct scsi_cmnd->serial_number needing to be an atomic_t.

Best,

--nab


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