linux-kernel - Re: TLB evaluation for Linux

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Message-Id: <200809021439.00484.nickpiggin@yahoo.com.au>
Date:	Tue, 2 Sep 2008 14:39:00 +1000
From:	Nick Piggin <nickpiggin@...oo.com.au>
To:	"CHADHA,VINEET" <vineet@....edu>
Cc:	linux-kernel@...r.kernel.org
Subject: Re: TLB evaluation for Linux

On Tuesday 02 September 2008 14:12, CHADHA,VINEET wrote:
> Hi,
>
> I have been working to evaluate TLB performance for Linux O/S and
> virtualized workloads(such as Xen) in a Full system simulator(e.g.
> simics). While my evaluation is in nascent stage, I do notice that
> most of the IPIs in multi-core environments cause complete TLB
> Flush.
>
> I want to evaluate cost of TLB shootdown including re-population
> vs. each entry shootdown (invlpg). While a similar study has been
> done in other kernels (e.g. L4 kernel), I am not aware if it has
> been done for Linux O/S.

This is a very interesting area to investigate. Do you have a link to
any of the existing studies?

> Are there hooks or patches to test or evaluate TLB performance.
> Specifically, I would like to know where to make changes in Linux
> kernel to support each entry shootdown.

The main thing I guess is to look at tlb_flush(), called by tlb_flush_mmu
when unmapping user virtual memory, which on x86 is going to call
flush_tlb_mm, which flushes the entire tlb.

It would need a bit of reworking of things in order to store the virtual
address corresponding to each page in the struct mmu_gather, and then
deciding to branch off to do multiple invlpg if you have only a small
number of pages to be flushed. I'd suggest the easiest way to get
something working on x86 would be to modify the asm-generic infrastructure
(ignore other architectures for the time being).

You will also have to rework the IPI flushing scheme so that it can handle
more than one flush_va for invlpg invalidations.

After you get all this done, you could also look at applying similar
heuristics to flush_tlb_range. This one should be much easier at this point,
but it is used in fewer places (eg. mprotect).
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