Re: [Xen-devel] [PATCH] Fix performance issue brought by TSC-sync logic

[Keir Fraser <keir.fraser@xxxxxxxxxxxxx>]

On 23/02/2009 00:21, "Yang, Xiaowei" <xiaowei.yang@xxxxxxxxx> wrote:

> Recently we found one performance bug when doing network test with VTd
> assigned devices - in some extreme case, the network performance in HVM
> using new Linux kernel could be 1/20 of native. Root cause is one of our
> sync-tsc-under-deep-C-state patches brings extra kilo-TSC drift between
> pCPUs and let check-tsc-sync logic in HVM failed. The result is the
> kernel fails to use platform timer (HPET, PMtimer) for gettimeofday
> instead of TSC and brings very frequent costly IOport access VMExit -
> triple per one call.
> 
> We provides below 2 patches to address the issue:

Patch 1 looks reasonable. Patch number 2 I'm less keen on, since patch 1
should suffice? Also I think regular re-sync across CPUs is a good idea
anyway. And that also reminds me -- isn't the CONSTANT_TSC logic in time.c
broken by host S3, and also by CPU hotplug? There's nothing to force sync of
AP TSC to BP TSC when an AP comes online after boot. Doesn't
init_percpu_time() need to handle that?

 -- Keir



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