Messages in this thread | | | Date | Thu, 26 Sep 2013 07:31:47 -0400 | From | Peter Hurley <> | Subject | Re: increased vmap_area_lock contentions on "n_tty: Move buffers into n_tty_data" |
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On 09/26/2013 03:33 AM, Andrew Morton wrote: > On Tue, 17 Sep 2013 20:22:42 -0400 Peter Hurley <peter@hurleysoftware.com> wrote: > >> Looking over vmalloc.c, the critical section footprint of the vmap_area_lock >> could definitely be reduced (even nearly eliminated), but that's a project for >> another day :) > > 20bafb3d23d10 ("n_tty: Move buffers into n_tty_data") switched a > kmalloc (which is very fast) to a vmalloc (which is very slow) without > so much as mentioning it in the changelog. This should have been > picked up at review, btw. > > Revert that part of the patch and the problem will be solved. > > If we are really really worried that a ~9k kmalloc might fail or will > be slow, then implement a fallback to vmalloc() if kmalloc(GFP_NOWARN) > failed. This kinda sucks, but is practical, but really should only be > done if necessary - ie, if problems with using plain old kmalloc are > demonstrable. > > Or just revert all of 20bafb3d23d10 - it was supposed to be a small > performance improvement but turned out to be a significant performance > loss. Therefore zap.
I have no particular objection to reverting the entire patch.
However, it's a mischaracterization to suggest that the reason is because vmalloc() is very slow; without reading /proc/meminfo, there is no performance loss.
IOW, the lock contention this patch precipitated needs to get fixed regardless.
Regards, Peter Hurley
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