Greg Kroah-Hartman
598cf27219
Link: https://lore.kernel.org/r/20260217200002.683975158@linuxfoundation.org Tested-by: Florian Fainelli <florian.fainelli@broadcom.com> Tested-by: Takeshi Ogasawara <takeshi.ogasawara@futuring-girl.com> Tested-by: Peter Schneider <pschneider1968@googlemail.com> Tested-by: Jon Hunter <jonathanh@nvidia.com> Tested-by: Salvatore Bonaccorso <carnil@debian.org> Tested-by: Brett A C Sheffield <bacs@librecast.net> Tested-by: Mark Brown <broonie@kernel.org> Tested-by: Luna Jernberg <droidbittin@gmail.com> Tested-by: Ronald Warsow <rwarsow@gmx.de> Tested-by: Justin M. Forbes <jforbes@fedoraproject.org> Tested-by: Ron Economos <re@w6rz.net> Tested-by: Miguel Ojeda <ojeda@kernel.org> Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
54 lines
2.4 KiB
Plaintext
54 lines
2.4 KiB
Plaintext
Intel Auto Counter Reload Support
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Support for Intel Auto Counter Reload in perf tools
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Auto counter reload provides a means for software to specify to hardware
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that certain counters, if supported, should be automatically reloaded
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upon overflow of chosen counters. By taking a sample only if the rate of
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one event exceeds some threshold relative to the rate of another event,
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this feature enables software to sample based on the relative rate of
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two or more events. To enable this, the user must provide a sample period
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term and a bitmask ("acr_mask") for each relevant event specifying the
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counters in an event group to reload if the event's specified sample
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period is exceeded.
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For example, if the user desires to measure a scenario when IPC > 2,
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the event group might look like the one below:
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perf record -e {cpu_atom/instructions,period=200000,acr_mask=0x2/, \
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cpu_atom/cycles,period=100000,acr_mask=0x3/} -- true
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In this case, if the "instructions" counter exceeds the sample period of
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200000, the second counter, "cycles", will be reset and a sample will be
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taken. If "cycles" is exceeded first, both counters in the group will be
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reset. In this way, samples will only be taken for cases where IPC > 2.
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The acr_mask term is a hexadecimal value representing a bitmask of the
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events in the group to be reset when the period is exceeded. In the
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example above, "instructions" is assigned an acr_mask of 0x2, meaning
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only the second event in the group is reloaded and a sample is taken
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for the first event. "cycles" is assigned an acr_mask of 0x3, meaning
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that both event counters will be reset if the sample period is exceeded
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first.
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ratio-to-prev Event Term
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------------------------
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To simplify this, an event term "ratio-to-prev" is provided which is used
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alongside the sample period term n or the -c/--count option. This would
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allow users to specify the desired relative rate between events as a
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ratio. Note: Both events compared must belong to the same PMU.
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The command above would then become
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perf record -e {cpu_atom/instructions/, \
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cpu_atom/cycles,period=100000,ratio-to-prev=0.5/} -- true
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ratio-to-prev is the ratio of the event using the term relative
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to the previous event in the group, which will always be 1,
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for a 1:0.5 or 2:1 ratio.
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To sample for IPC < 2 for example, the events need to be reordered:
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perf record -e {cpu_atom/cycles/, \
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cpu_atom/instructions,period=200000,ratio-to-prev=2.0/} -- true
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