Re: [PATCH 2/2] libata: micro-optimize tag allocation

[Shaohua Li <shli@fb.com>]

On Fri, Jan 16, 2015 at 03:49:07PM -0800, Dan Williams wrote:
> On Fri, Jan 16, 2015 at 3:31 PM, Shaohua Li <shli@fb.com> wrote:
> > On Fri, Jan 16, 2015 at 03:13:08PM -0800, Dan Williams wrote:
> >> Jens notes, "libata tag allocator sucks. Like seriously sucks, it's
> >> almost a worst case implementation."  Previously I thought SATA mmio
> >> latency dominated performance profiles, but as Tejun notes:
> >>
> >>   "Hmmm... one problem with the existing tag allocator in ata is that
> >>    it's not very efficient which actually shows up in profile when libata
> >>    is used with a very zippy SSD.  Given that ata needs a different
> >>    allocation policies anyway maybe the right thing to do is making the
> >>    existing allocator suck less."
> >>
> >> So replace it with a naive enhancement that also supports the existing
> >> quirks.  Hopefully, soon to be replaced by Shaohua's patches [1], but
> >> those do not yet support the quirk needed by sil24 (ATA_FLAG_LOWTAG)
> >> [2].
> >>
> >> [1]: https://urldefense.proofpoint.com/v1/url?u=http://marc.info/?l%3Dlinux-ide%26m%3D142137195324687%26w%3D2&k=ZVNjlDMF0FElm4dQtryO4A%3D%3D%0A&r=3RFlL12A7nwmLRXunVJq2g%3D%3D%0A&m=5iyIBsm0XLUeBFFdEfPvaP6LL8KiN2TcuEGgJH6RzTc%3D%0A&s=cea914a77883ea668400a1f19621d1241cab16f6a4c0e68d0749de4c2448cdb1
> >> [2]: https://urldefense.proofpoint.com/v1/url?u=https://bugzilla.kernel.org/show_bug.cgi?id%3D87101&k=ZVNjlDMF0FElm4dQtryO4A%3D%3D%0A&r=3RFlL12A7nwmLRXunVJq2g%3D%3D%0A&m=5iyIBsm0XLUeBFFdEfPvaP6LL8KiN2TcuEGgJH6RzTc%3D%0A&s=82d11bc720e9f0cbd0ad6aa7e8ece18315ac1e4f97cb02d49460893e5006228e
> >
> > with my patch, we can fix this as:
> >
> >
> > diff --git a/drivers/ata/sata_sil24.c b/drivers/ata/sata_sil24.c
> > index d81b20d..5242897 100644
> > --- a/drivers/ata/sata_sil24.c
> > +++ b/drivers/ata/sata_sil24.c
> > @@ -388,6 +388,7 @@ static struct scsi_host_template sil24_sht = {
> >         .can_queue              = SIL24_MAX_CMDS,
> >         .sg_tablesize           = SIL24_MAX_SGE,
> >         .dma_boundary           = ATA_DMA_BOUNDARY,
> > +       .tag_alloc_policy       = BLK_TAG_ALLOC_FIFO,
> >  };
> >
> >  static struct ata_port_operations sil24_ops = {
> 
> Ok, thanks for that.
> 
> We still need patch1 as the minimal fix for the regression, agreed?

The BLK_TAG_ALLOC_FIFO will make blk/blk-mq tag allocation allocates
lowest tag (eg, FIFO). So I thought it already fixes the sil24 bug, if I
understand the bug clearly.

Thanks,
Shaohua