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SubjectRe: [PATCH 3/3] x86: UV2 BAU hang workarounds
On Mon, Jun 25, 2012 at 12:03:21PM +0200, Ingo Molnar wrote:
>
> * Cliff Wickman <cpw@sgi.com> wrote:
>
> > On SGI's UV2 the BAU (Broadcast Assist Unit) driver can hang under a
> > heavy load. To cure this:
> >
> > - Disable the UV2 extended status mode (see UV2_EXT_SHFT), as this
> > mode changes BAU behavior in more ways then just delivering an extra bit
> > of status. Revert status to just two meaningful bits, like UV1.
> > - Use no IPI-style resets on UV2. Just give up the request for whatever the
> > reason it failed and let it be accomplished with the legacy IPI method.
> > - Use no alternate sending descriptor (the former UV2 workaround
> > bcp->using_desc and handle_uv2_busy() stuff). Just disable the use of the
> > BAU for a period of time in favor of the legacy IPI method when the h/w bug
> > leaves a descriptor busy.
> > -- new tunable: giveup_limit determines the threshold at which a hub is
> > so plugged that it should do all requests with the legacy IPI method for a
> > period of time
> > -- generalize disable_for_congestion() (renamed disable_for_period()) for
> > use whenever a hub should avoid using the BAU for a period of time
> >
> > Misc:
> > - fix find_another_by_swack(), which is part of the UV2 bug workaround
> > - correct and clarify the statistics (new stats s_overipilimit s_giveuplimit
> > s_enters s_ipifordisabled s_plugged s_congested)
>
> Sigh, it looks like something that ought to be 7 successive,
> easy to review commits got mixed up into a single, huge, hard to
> review commit. How did that happen?
>
> Thanks,
>
> Ingo

Hi Ingo,

Yes, admittedly large.
This patch was the 'bottom line' of a great deal of experimentation on
how to work around some hardware problems with the bau. This is what
remains after pulling out the unnecessary or unhelpful attempts.
I could break it up for review purposes, if you think anyone would
want to examine each component.
You sound like you're willing to spend that time and effort. Yes?

-Cliff

--
Cliff Wickman
SGI
cpw@sgi.com
(651) 683-3824


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