linux-kernel - Re: [PATCHv4] SGI UV: TLB shootdown using broadcast assist unit

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Message-Id: <200806122318.45397.nickpiggin@yahoo.com.au>
Date:	Thu, 12 Jun 2008 23:18:45 +1000
From:	Nick Piggin <nickpiggin@...oo.com.au>
To:	Cliff Wickman <cpw@....com>
Cc:	linux-kernel@...r.kernel.org, mingo@...e.hu
Subject: Re: [PATCHv4] SGI UV: TLB shootdown using broadcast assist unit

On Thursday 12 June 2008 22:56, Cliff Wickman wrote:
> Hi Nick,
>
> On Thu, Jun 12, 2008 at 10:35:29PM +1000, Nick Piggin wrote:
> > On Thursday 12 June 2008 22:23, Cliff Wickman wrote:

> > For someone not too familiar with low level x86 (or UV) code, can
> > you explain why you are hooking at this point? I mean, what it
> > looks like is either a performance improvement, or for some reason
> > UV does not support send_IPI_mask out to CPUs "not on the local node".
>
> Yes, a performance improvement.  The UV machine has hardware for
> broadcasting messages to a set of nodes (represented in a bit mask).  The
> messages will raise interrupts at each of the target nodes and provide
> the message - all in one step.
> (IPI is supported.  In fact this patch falls back to the IPI method
>  if all the cpus on the remote nodes do not respond.)

Thanks, that makes it perfectly clear to me now (the intent, not
the details of the code :))

So long as this raises a maskable interrupt on each target CPU, it
doesn't break x86's lockless get_user_pages :)


> > If the former, what sort of improvement to you expect / see?
>
> Good question.  The hardware does not exist yet.  But using IPI there
> would be one set of packets exchanged to deliver the interrupts and
> another set to pull over the flush address, just to start the operation.
> I expect the improvement to be significant.

Ah, so you can send a small message with the IPI, and that can be
decoded and used by the target without invoking the cc protocol.
Seems like pretty sweet functionality.

I guess TLB flushing is an obvious candidate, but it could be
quite useful for other operations as well. I wonder if it couldn't
be used to create a slightly more advanced API (than send_IPI)
which other platforms can just implement using cache coherency for
the payload...

For example, some classes of smp_call_function could use this too.

But for now I don't see anything wrong with getting this patch
upstream and looking to generalise it later.
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