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.
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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