Messages in this thread | | | Date | Thu, 12 Jun 2008 09:28:52 -0500 | From | Cliff Wickman <> | Subject | Re: [PATCHv4] SGI UV: TLB shootdown using broadcast assist unit |
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Hi Nick,
On Thu, Jun 12, 2008 at 11:18:45PM +1000, Nick Piggin wrote: > 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.
Jack Steiner's thought as well. But I haven't considered any yet. If you care to nominate any other such uses for this hardware mechanism I'd like to hear your ideas.
> > But for now I don't see anything wrong with getting this patch > upstream and looking to generalise it later.
-Cliff -- Cliff Wickman Silicon Graphics, Inc. cpw@sgi.com (651) 683-3824
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