Re: [Xen-devel] [PATCH for-4.12] x86: Improve the efficiency of domain_relinquish_resources()

[Julien Grall <julien.grall@xxxxxxx>]

Hi Andrew,

On 2/21/19 12:22 PM, Andrew Cooper wrote:
> pci_release_devices() takes the global PCI lock.  Once pci_release_devices()
> has completed, it will be called redundantly each time paging_teardown() and
> vcpu_destroy_pagetables() continue.
>
> This is liable to be millions of times for a reasonably sized guest, and is a
> serialising bottleneck now that domain_kill() can be run concurrently on
> different domains.
>
> Instead of propagating the opencoding of the relinquish state machine, take
> the opportunity to clean it up.
>
> Leave a proper set of comments explaining that domain_relinquish_resources()
> implements a co-routine.  Introduce a documented PROGRESS() macro to avoid
> latent bugs such as the RELMEM_xen case, and make the new PROG_* states
> private to domain_relinquish_resources().
>
> Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
> ---
> CC: Jan Beulich <JBeulich@xxxxxxxx>
> CC: Wei Liu <wei.liu2@xxxxxxxxxx>
> CC: Roger Pau Monné <roger.pau@xxxxxxxxxx>
> CC: Juergen Gross <jgross@xxxxxxxx>
> CC: Stefano Stabellini <sstabellini@xxxxxxxxxx>
> CC: Julien Grall <julien.grall@xxxxxxx>
>
> So I know Xen 4.12 isn't going to crash and burn without this change, but I
> also can't un-see the unnecessary global PCI lock contention.  In terms of
> risk, this is extremely low - this function has complete coverage in testing,
> and its behaviour isn't changing dramatically.
>
> ARM: There are no problems, latent or otherwise, with your version of
> domain_relinquish_resources(), but I'd recommend the same cleanup in due
> course.
I will add in my todo list of cleanup!

Cheers,

--
Julien Grall

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