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

[Anthony PERARD <anthony.perard@xxxxxxxxxx>]

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.

Thanks,

-- 
Anthony PERARD