Re: increased vmap_area_lock contentions on "n_tty: Move buffersinto n_tty_data"

[Peter Hurley]

On 09/26/2013 11:04 AM, Greg KH wrote:
> On Thu, Sep 26, 2013 at 07:31:47AM -0400, Peter Hurley wrote:
> > On 09/26/2013 03:33 AM, Andrew Morton wrote:
> > > On Tue, 17 Sep 2013 20:22:42 -0400 Peter Hurley <peter@xxxxxxxxxxxxxxxxxx> 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.
>
> How about just switching the call to vmalloc to kmalloc?  Yes, it's a
> larger size that is being allocated here, but we were allocating that
> much memory anyway before, so it should be the same "speed", if not
> faster than before (1 call to kmalloc instead of 3).

The allocation itself isn't performance-critical. The speed difference
between 1 kmalloc and 3 kmallocs here will be unmeasurable from any
user-space test.

And the only reason vmalloc has any measurable impact stems from the way
reads of /proc/meminfo behave (to which there are a number of appropriate
solutions).

The issue with a single large kmalloc is that it may fail where
3 separate, page-or-less kmallocs would not have.

Regards,
Peter Hurley
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