Messages in this thread | | | From | Nick Piggin <> | Subject | Re: [PATCHv4] SGI UV: TLB shootdown using broadcast assist unit | Date | Thu, 12 Jun 2008 23:18:45 +1000 |
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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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