Re: [PATCH 1/8] scsi: Drop struct Scsi_Host->host_lock around SHT->queuecommand()

[Joe Eykholt]

On 9/17/10 9:37 AM, 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);
>> }

A tiny trick I like to use is to start a serial number at 1 and
increment by 2 so its always odd and then never wraps to 0.
That eliminates the check for 0 (and the curly brackets).

> 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.
> 
> I will take a look at this conversion and respin a complete set of
> patches for review a bit later today.
> 
> Thanks!
> 
> --nab
> 
>>
>> 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
>>
>>
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