Re: [PATCH] xl: relax freemem()'s retry calculation

[Anthony PERARD <anthony.perard@xxxxxxxxxx>]

On Tue, Jul 12, 2022 at 09:01:48AM +0200, Jan Beulich wrote:
> On 11.07.2022 18:21, Anthony PERARD wrote:
> > On Fri, Jul 08, 2022 at 03:39:38PM +0200, Jan Beulich wrote:
> >> While in principle possible also under other conditions as long as other
> >> parallel operations potentially consuming memory aren't "locked out", in
> >> particular with IOMMU large page mappings used in Dom0 (for PV when in
> >> strict mode; for PVH when not sharing page tables with HAP) ballooning
> >> out of individual pages can actually lead to less free memory available
> >> afterwards. This is because to split a large page, one or more page
> >> table pages are necessary (one per level that is split).
> >>
> >> When rebooting a guest I've observed freemem() to fail: A single page
> >> was required to be ballooned out (presumably because of heap
> >> fragmentation in the hypervisor). This ballooning out of a single page
> >> of course went fast, but freemem() then found that it would require to
> >> balloon out another page. This repeating just another time leads to the
> >> function to signal failure to the caller - without having come anywhere
> >> near the designated 30s that the whole process is allowed to not make
> >> any progress at all.
> >>
> >> Convert from a simple retry count to actually calculating elapsed time,
> >> subtracting from an initial credit of 30s. Don't go as far as limiting
> >> the "wait_secs" value passed to libxl_wait_for_memory_target(), though.
> >> While this leads to the overall process now possibly taking longer (if
> >> the previous iteration ended very close to the intended 30s), this
> >> compensates to some degree for the value passed really meaning "allowed
> >> to run for this long without making progress".
> >>
> >> Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
> >> ---
> >> I further wonder whether the "credit expired" loop exit wouldn't better
> >> be moved to the middle of the loop, immediately after "return true".
> >> That way having reached the goal on the last iteration would be reported
> >> as success to the caller, rather than as "timed out".
> > 
> > That would sound like a good improvement to the patch.
> 
> Oh. I would have made it a separate one, if deemed sensible. Order
> shouldn't matter as I'd consider both backporting candidates.

OK.

For this patch:
Reviewed-by: Anthony PERARD <anthony.perard@xxxxxxxxxx>

Thanks,

-- 
Anthony PERARD