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SubjectRe: [RFC][PATCH] flush_icache_user_range
On Thu, Nov 22, 2001 at 10:43:48PM +1100, Paul Mackerras wrote:
> The patch below changes access_one_page in kernel/ptrace.c to use a
> new function, flush_icache_user_range, instead of flush_icache_page as
> at present. The reason for making this change is that
> flush_icache_page is also called in do_no_page and do_swap_page, where
> it does a fundamentally different job. Decoupling the two makes it
> possible to improve performance, because we can make flush_icache_page
> do the flush only when needed.
> This patch particularly affects alpha: for alpha I renamed the
> existing flush_icache_page to flush_icache_user_range and made
> flush_icache_page a no-op. This is based on a suggestion from
> Andrea and the comments in the code. I would be very interested to
> hear how it affects alpha as regards stability and performance.
> For PPC, I have added a flush_icache_user_range that only flushes the
> cachelines that have been modified. I have a more extensive
> cache-flush avoidance patch in the works that depends on this one and
> also needs modifications to clear_user_page and copy_user_page. This
> all gives us a substantial performance increase on PPC.
> For all the other architectures I have made flush_icache_user_range
> the same as flush_icache_page, i.e. a no-op for most architectures.
> So this patch should have zero impact except on alpha and PPC, and on
> alpha it should improve performance.
> Note that flush_icache_user_range is different from flush_icache_page
> now in that flush_icache_user_range is only called when the page has
> been modified, so we know we do have to flush (if the icache doesn't
> snoop stores by the cpu). In contrast, flush_icache_page is called in
> situations where the page often (usually?) is unmodified, so it makes
> sense to try to work out whether the flush is actually needed.
> Comments? Linus, would you be willing to apply this?

I will try to give it a spin, it should be a very nice speedup for
alpha, we were wasting an huge amount of asn (and possibly of IPI too
with threads) during major faults and this will optimize it away.

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