Date:   Wed, 19 Sep 2018 10:29:05 -0700
From:   Reinette Chatre <reinette.chatre@...el.com>
To:     tglx@...utronix.de, fenghua.yu@...el.com, tony.luck@...el.com,
        peterz@...radead.org, mingo@...hat.com, acme@...nel.org
Cc:     gavin.hindman@...el.com, jithu.joseph@...el.com,
        dave.hansen@...el.com, hpa@...or.com, x86@...nel.org,
        linux-kernel@...r.kernel.org,
        Reinette Chatre <reinette.chatre@...el.com>
Subject: [PATCH V5 0/6] perf and x86/intel_rdt: Fix lack of coordination with perf

Dear Maintainers,

This new series fixing the lack of coordination between the
pseudo-locking measurement code and perf addresses all feedback received
up to and including V4.

Changes since V4:
- Move the helper to obtain the performance counter index
  (x86_perf_rdpmc_index()) to arch/x86/events/core.c with an extern
  declaration in arch/x86/include/asm/perf_event.h. Please do note that
  this change introduces a checkpatch.pl warning that was discussed
  and found to be a false positive in:
  http://lkml.kernel.org/r/20180919091759.GZ24124@hirez.programming.kicks-ass.net
  The new warning is:
  > CHECK: extern prototypes should be avoided in .h files
  > #66: FILE: arch/x86/include/asm/perf_event.h:273:
  > +extern int x86_perf_rdpmc_index(struct perf_event *event);

- Change the x86_perf_rdpmc_index() comment slightly:
  "... could be checked for validity ..." -> "... should be checked for
  validity ...". No code changes.
- Add comments to the measurement routines to document that on measurement
  failure zeroes will be written to the trace buffer instead of leaving an
  empty trace buffer. No code changes.

Changes since V3:
- The kbuild test robot reported a build issue that at first seems to be
  related to "make ARCH=i386" but was actually a result of the kernel
  configuration used not having CONFIG_TRACEPOINTS set.
  The consequence was that include/linux/perf_event.h was not being included
  via the tracing code (from include/trace/define_trace.h) and we thus
  encounter the build failure where struct perf_event_attr is unknown.
  Fix this by explicitly including perf_event.h

- Below is verbatim from V3 submission (except for diffstat below) -

Changes since V2:
- Move the helper to obtain the performance counter index to
  include/linux/perf_event.h. The request was actually to move this helper
  to arch/x86/include/asm/perf_event.h - but doing so would be more
  involved since this header file does not know about struct perf_event
  that is used by this helper. There was no response for further
  clarification of the request to move this helper so I proceeded to move it
  to include/linux/perf_event.h instead.
- Change name of helper to obtain the index to perf_rdpmc_index() - the
  original request was to name it x86_perf_rdpmc_index() but this seems to
  be tied to the suggested header location. With the header location of
  include/linux/perf_event.h the name perf_rdpmc_index() seems to fit
  better with the new destination. There was no response for further
  clarification of the naming change request so I proceeded with the change.
- Replace all local register variables used in the measurement routines
  with local variables using READ_ONCE().
- The removal of local register variables also enable us to replace the
  direct __wrmsr() with wrmsr().
- Merge the L2 and L3 measurement routines following Peter's suggested
  framework.
- Do not copy the text from SDM that refers to serializing instructions.
- Include another LFENCE call after loop reading pseudo-locked memory.

The above addresses all feedback received for V2. The one unanswered
question that remains following the review is why the memory reading
was done with asm: the reason I did so was to avoid any compiler
optimizations while constraining the code exactly to what needed to be
measured. By using the asm instruction I am able to use a single instruction
to read a cache line into a register. To me this seemed the most constrained
way to measure if a cache line is in the cache.

This is the second attempt at fixing the lack of coordination between the
pseudo-locking measurement code and perf. Thank you very much for your
feedback on the first version. The entire solution, including the cover
letter, has been reworked based on your feedback, while submitted as a V2,
none of the patches from V1 remained.

Changes since V1:
- Use in-kernel interface to perf.
- Do not write directly to PMU registers.
- Do not introduce another PMU owner. perf maintains role as performing
  resource arbitration for PMU.
- User space is able to use perf and resctrl at the same time.
- event_base_rdpmc is accessed and used only within an interrupts
  disabled section.
- Internals of events are never accessed directly, inline function used.
- Due to "pinned" usage the scheduling of event may have failed.  Error
  state is checked in recommended way and have a credible error
  handling.
- use X86_CONFIG

This code is based on the x86/cache branch of tip.git

The success of Cache Pseudo-Locking, as measured by how many cache lines
from a physical memory region has been locked to cache, can be measured
via the use of hardware performance events. Specifically, the number of
cache hits and misses reading a memory region after it has been
pseudo-locked to cache. This measurement is triggered via the resctrl
debugfs interface.

The current solution accesses performance counters and their configuration
registers directly without coordination with other performance event users
(perf).
Two of the issues that exist with the current solution:
- By writing to the performance monitoring registers directly a new owner
  for these resources is introduced. The perf infrastructure already exist
  to perform resource arbitration and the in-kernel infrastructure should
  be used to do so.
- The current lack of coordination with perf will have consequences any time
  two users, for example perf and cache pseudo-locking, attempt to do any
  kind of measurement at the same time.

In this series the measurement of Cache Pseudo-Lock regions is moved to use
the in-kernel interface to perf. During the rework of the measurement
function the L2 and L3 cache measurements are separated to avoid the
additional code needed to decide on which measurement causing unrelated
cache hits and misses.

Your feedback on this work will be greatly appreciated.

Reinette

Reinette Chatre (6):
  perf/core: Add sanity check to deal with pinned event failure
  perf/x86: Add helper to obtain performance counter index
  x86/intel_rdt: Remove local register variables
  x86/intel_rdt: Create required perf event attributes
  x86/intel_rdt: Use perf infrastructure for measurements
  x86/intel_rdt: Re-enable pseudo-lock measurements

 Documentation/x86/intel_rdt_ui.txt          |  22 +-
 arch/x86/events/core.c                      |  21 ++
 arch/x86/include/asm/perf_event.h           |   1 +
 arch/x86/kernel/cpu/intel_rdt_pseudo_lock.c | 372 ++++++++++++--------
 kernel/events/core.c                        |   6 +
 5 files changed, 261 insertions(+), 161 deletions(-)

-- 
2.17.0

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