From: "Jan Beulich" <JBeulich@suse.com> To: Joao Martins <email@example.com> Cc: Andrew Cooper <firstname.lastname@example.org>, Wei Liu <email@example.com>, Dario Faggioli <firstname.lastname@example.org>, xen-devel <email@example.com> Subject: Re: [PATCH] x86/time: use correct (local) time stamp in constant-TSC calibration fast path Date: Thu, 09 Jun 2016 06:57:02 -0600 [thread overview] Message-ID: <575983BE02000078000F37BB@prv-mh.provo.novell.com> (raw) In-Reply-To: <57595D07.firstname.lastname@example.org> >>> On 09.06.16 at 14:11, <email@example.com> wrote: > On 06/09/2016 01:01 PM, Jan Beulich wrote: >> This looks like a copy and paste mistake in commit 1b6a99892d ("x86: >> Simpler time handling when TSC is constant across all power saving >> states"), responsible for occasional many-microsecond cross-CPU skew of >> what NOW() returns. >> >> Also improve the correlation between local TSC and stime stamps >> obtained at the end of the two calibration handlers: Compute the stime >> one from the TSC one, instead of doing another rdtsc() for that >> compuation. >> >> Signed-off-by: Jan Beulich <firstname.lastname@example.org> >> --- >> As to 4.7 inclusion: This of course looks like a pretty blatant mistake >> that has been there for many years (and hence many releases). There's >> certainly non-zero risk that I'm overlooking something here (despite >> Joao apparently having come to the same conclusion), so I can't really >> make up my mind on whether to request this patch to be put there right >> away, or rather having linger in -unstable for a while. >> > Initially I thought of this as a fix too, but then wouldn't having > t->stime_local_stamp be c->stime_local_stamp, render no use to the > platform timer reads done on calibration? Unless we would change > update_vcpu_system to use stime_master_stamp instead? > stime_master_stamp field isn't used anywhere other than the dom0 injected > cpu_frequency_change or when at boot seeding the cpu_time struct on > init_percpu_time (and the already mentioned use on local_time_calibration) ? > init_percpu_time also takes a different read of the platform timer per > cpu and could probably be inherited by a read done on the boot processor > and written on remaining CPUs, so that all would start from the same stamp. > IOW - this sounds like time we are turning stime to be totally TSC except > when initially seeding each cpu_time? Did you also look at the "slow" path in local_time_calibration()? That does use the master stamp. So in effect for the fast path the patch changes the situation from c->stime_local_stamp being effectively unused to c->stime_master_stamp being so. In the former case, if that really hadn't been a typo, deleting the write of that field from time_calibration_std_rendezvous() would have made sense, as get_s_time() certainly is more overhead than the simply memory read and write needed for keeping c->stime_master_stamp up to date (despite not being used). Jan _______________________________________________ Xen-devel mailing list Xenemail@example.com http://lists.xen.org/xen-devel
next prev parent reply other threads:[~2016-06-09 12:57 UTC|newest] Thread overview: 11+ messages / expand[flat|nested] mbox.gz Atom feed top 2016-06-09 12:01 Jan Beulich 2016-06-09 12:10 ` Andrew Cooper 2016-06-09 12:11 ` Joao Martins 2016-06-09 12:57 ` Jan Beulich [this message] 2016-06-09 15:00 ` Joao Martins 2016-06-09 15:52 ` Jan Beulich 2016-06-09 18:19 ` Joao Martins 2016-06-10 6:59 ` Jan Beulich 2016-06-10 9:29 ` Jan Beulich 2016-06-10 17:07 ` Joao Martins 2016-06-09 12:12 ` Wei Liu
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