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Peter Zijlstra 09f5e7dc7a perf: Fix perf_event_read_local() time 
Time readers that cannot take locks (due to NMI etc..) currently make
use of perf_event::shadow_ctx_time, which, for that event gives:

  time' = now + (time - timestamp)

or, alternatively arranged:

  time' = time + (now - timestamp)

IOW, the progression of time since the last time the shadow_ctx_time
was updated.

There's problems with this:

 A) the shadow_ctx_time is per-event, even though the ctx_time it
    reflects is obviously per context. The direct concequence of this
    is that the context needs to iterate all events all the time to
    keep the shadow_ctx_time in sync.

 B) even with the prior point, the context itself might not be active
    meaning its time should not advance to begin with.

 C) shadow_ctx_time isn't consistently updated when ctx_time is

There are 3 users of this stuff, that suffer differently from this:

 - calc_timer_values()
   - perf_output_read()
   - perf_event_update_userpage()	/* A */

 - perf_event_read_local()		/* A,B */

In particular, perf_output_read() doesn't suffer at all, because it's
sample driven and hence only relevant when the event is actually
running.

This same was supposed to be true for perf_event_update_userpage(),
after all self-monitoring implies the context is active *HOWEVER*, as
per commit f792565326 ("perf/core: fix userpage->time_enabled of
inactive events") this goes wrong when combined with counter
overcommit, in that case those events that do not get scheduled when
the context becomes active (task events typically) miss out on the
EVENT_TIME update and ENABLED time is inflated (for a little while)
with the time the context was inactive. Once the event gets rotated
in, this gets corrected, leading to a non-monotonic timeflow.

perf_event_read_local() made things even worse, it can request time at
any point, suffering all the problems perf_event_update_userpage()
does and more. Because while perf_event_update_userpage() is limited
by the context being active, perf_event_read_local() users have no
such constraint.

Therefore, completely overhaul things and do away with
perf_event::shadow_ctx_time. Instead have regular context time updates
keep track of this offset directly and provide perf_event_time_now()
to complement perf_event_time().

perf_event_time_now() will, in adition to being context wide, also
take into account if the context is active. For inactive context, it
will not advance time.

This latter property means the cgroup perf_cgroup_info context needs
to grow addition state to track this.

Additionally, since all this is strictly per-cpu, we can use barrier()
to order context activity vs context time.

Fixes: 7d9285e82d ("perf/bpf: Extend the perf_event_read_local() interface, a.k.a. "bpf: perf event change needed for subsequent bpf helpers"")
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Tested-by: Song Liu <song@kernel.org>
Tested-by: Namhyung Kim <namhyung@kernel.org>
Link: https://lkml.kernel.org/r/YcB06DasOBtU0b00@hirez.programming.kicks-ass.net
History 2022-01-18 12:09:47 +01:00
arch Rework of the MSI interrupt infrastructure: 2022-01-13 09:05:29 -08:00
block Convert xfs/iomap to use folios 2022-01-12 12:51:41 -08:00
certs certs: Add support for using elliptic curve keys for signing modules 2021-08-23 19:55:42 +03:00
crypto tpmdd updates for Linux v5.17 2022-01-11 12:58:41 -08:00
Documentation - qcom: misc updates to qcom-ipcc driver 2022-01-13 11:19:07 -08:00
drivers - qcom: misc updates to qcom-ipcc driver 2022-01-13 11:19:07 -08:00
fs dax + libnvdimm for v5.17 2022-01-12 15:46:11 -08:00
include perf: Fix perf_event_read_local() time 2022-01-18 12:09:47 +01:00
init Peter Zijlstra says: 2022-01-12 16:26:58 -08:00
ipc shm: extend forced shm destroy to support objects from several IPC nses 2021-11-20 10:35:54 -08:00
kernel perf: Fix perf_event_read_local() time 2022-01-18 12:09:47 +01:00
lib Convert much of the page cache to use folios 2022-01-12 12:37:02 -08:00
LICENSES LICENSES/LGPL-2.1: Add LGPL-2.1-or-later as valid identifiers 2021-12-16 14:33:10 +01:00
mm dax + libnvdimm for v5.17 2022-01-12 15:46:11 -08:00
net TTY/Serial driver updates for 5.17-rc1 2022-01-12 11:21:52 -08:00
samples - Get rid of all the .fixup sections because this generates 2022-01-12 16:31:19 -08:00
scripts - Get rid of all the .fixup sections because this generates 2022-01-12 16:31:19 -08:00
security fs.idmapped.v5.17 2022-01-11 14:26:55 -08:00
sound sound fixes for 5.16-rc7 2021-12-23 09:55:58 -08:00
tools - Get rid of all the .fixup sections because this generates 2022-01-12 16:31:19 -08:00
usr initramfs: Check timestamp to prevent broken cpio archive 2021-10-24 13:48:40 +09:00
virt Peter Zijlstra says: 2022-01-12 16:26:58 -08:00
.clang-format genirq/msi: Make interrupt allocation less convoluted 2021-12-16 22:22:20 +01:00
.cocciconfig scripts: add Linux .cocciconfig for coccinelle 2016-07-22 12:13:39 +02:00
.get_maintainer.ignore Opt out of scripts/get_maintainer.pl 2019-05-16 10:53:40 -07:00
.gitattributes .gitattributes: use 'dts' diff driver for dts files 2019-12-04 19:44:11 -08:00
.gitignore .gitignore: ignore only top-level modules.builtin 2021-05-02 00:43:35 +09:00
.mailmap media updates for v5.17-rc1 2022-01-10 18:55:43 -08:00
COPYING COPYING: state that all contributions really are covered by this file 2020-02-10 13:32:20 -08:00
CREDITS MAINTAINERS: Move Daniel Drake to credits 2021-09-21 08:34:58 +03:00
Kbuild kbuild: rename hostprogs-y/always to hostprogs/always-y 2020-02-04 01:53:07 +09:00
Kconfig kbuild: ensure full rebuild when the compiler is updated 2020-05-12 13:28:33 +09:00
MAINTAINERS Updates for the time(r) subsystem: 2022-01-13 09:02:27 -08:00
Makefile - Get rid of all the .fixup sections because this generates 2022-01-12 16:31:19 -08:00
README Drop all 00-INDEX files from Documentation/ 2018-09-09 15:08:58 -06:00

README 

Linux kernel
============

There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.

In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``.  The formatted documentation can also be read online at:

    https://www.kernel.org/doc/html/latest/

There are various text files in the Documentation/ subdirectory,
several of them using the Restructured Text markup notation.

Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.