From: Nitesh Narayan Lal <email@example.com>
To: firstname.lastname@example.org, email@example.com,
firstname.lastname@example.org, email@example.com, firstname.lastname@example.org,
Subject: Possible regression in cpuacct.stats system time
Date: Mon, 4 Jan 2021 16:24:30 -0500 [thread overview]
Message-ID: <email@example.com> (raw)
Last year I reported an issue of "suspicious RCU usage"  with the debug
kernel which was fixed with the patch:
87fa7f3e98 "x86/kvm: Move context tracking where it belongs"
Recently I have come across a possible regression because of this
patch in the cpuacct.stats system time.
With the latest upstream kernel (5.11-rc2) when we set up a VM and start
observing the system time value from cpuacct.stat then it is significantly
higher than value reported with the kernel that doesn't have the
previously mentioned patch.
For instance, the following are the values of cpuacct.stats right after the
VM bring up completion for two cases:
with a kernel that has the patch-
with the patch reverted-
FWIU the reason behind this increase is the moving of guest_exit_irqoff()
to its proper location (near vmexit). This leads to the accounting
of instructions that were previously accounted into the guest context as a
part of the system time.
IMO this should be an expected behavior after the previously mentioned
change. Is that a right conclusion or I am missing something here?
Another question that I have is about the patch
d7a08882a0 "KVM: x86: Unconditionally enable irqs in guest context"
considering we are enabling irqs early now in the code path, do we still
need this patch?
next reply other threads:[~2021-01-04 21:26 UTC|newest]
Thread overview: 2+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-01-04 21:24 Nitesh Narayan Lal [this message]
2021-01-05 7:27 ` Possible regression in cpuacct.stats system time Paolo Bonzini
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
* If your mail client supports setting the In-Reply-To header
via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).