On Fri, May 14, 2021 at 04:50:26PM +0200, Uladzislau Rezki wrote:
> > > > However, the high-order path also looks suspicious. area->nr_pages is
> > > > advanced before the allocation attempt so in the event alloc_pages_node()
> > > > returns NULL prematurely, area->nr_pages does not reflect the number of
> > > > pages allocated so that needs examination.
> > > >
> > > <snip>
> > > for (area->nr_pages = 0; area->nr_pages < nr_small_pages;
> > > area->nr_pages += 1U << page_order) {
> > > <snip>
> > >
> > > if alloc_pages_node() fails we break the loop. area->nr_pages is initialized
> > > inside the for(...) loop, thus it will be zero if the single page allocator
> > > fails on a first iteration.
> > >
> > > Or i miss your point?
> > >
> >
> > At the time of the break, area->nr_pages += 1U << page_order happened
> > before the allocation failure happens. That looks very suspicious.
> >
> The "for" loop does not work that way. If you break the loop the
> "area->nr_pages += 1U << page_order" or an "increment" is not increased.
> It is increased only after the body of the "for" loop executes and it
> goes to next iteration.
>
Yeah, I don't know what I was thinking -- not properly anyway.
> > > As for workloads. Most of them which are critical to time and latency. For
> > > example audio/video, especially in the mobile area. I did a big rework of
> > > the KVA allocator because i found it not optimal to allocation time.
> > >
> >
> > Can you give an example benchmark that triggers it or is it somewhat
> > specific to mobile platforms with drivers that use vmalloc heavily?
> >
>
> See below an example of audio glitches. That was related to our phones
> and audio workloads:
>
> # Explanation is here
> wget ftp://vps418301.ovh.net/incoming/analysis_audio_glitches.txt
>
> # Audio 10 seconds sample is here.
> # The drop occurs at 00:09.295 you can hear it
> wget ftp://vps418301.ovh.net/incoming/tst_440_HZ_tmp_1.wav
>
> Apart of that a slow allocation can course two type of issues. First one
> is direct. When for example a high-priority RT thread does some allocation
> to bypass data to DSP. Long latency courses a delay of data to be passed to
> DSP. This is drivers area.
>
> Another example is when a task is doing an allocation and the RT task is
> placed onto a same CPU. In that case a long preemption-off(milliseconds)
> section can lead the RT task for starvation. For mobile devices it is UI
> stack where RT tasks are used. As a result we face frame drops.
>
> All such issues have been solved after a rework:
>
> wget ftp://vps418301.ovh.net/incoming/Reworking_of_KVA_allocator_in_Linux_kernel.pdf
>
Thanks. That was enough for me to search to see what sort of general
workload would be affected. Mostly it's driver specific. A lot of the users
that would be potentially hot are already using kvmalloc so probably not
worth the effort so test_vmalloc.sh makes sense.
--
Mel Gorman
SUSE Labs
