Date:   Wed, 1 Apr 2020 16:01:06 +0200
From:   Jiri Olsa <jolsa@...hat.com>
To:     Alexey Budankov <alexey.budankov@...ux.intel.com>
Cc:     Arnaldo Carvalho de Melo <acme@...hat.com>,
        Namhyung Kim <namhyung@...nel.org>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>,
        Andi Kleen <ak@...ux.intel.com>,
        linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v1 0/8] perf: support resume and pause commands in stat
 and record modes

On Fri, Mar 27, 2020 at 11:34:54AM +0300, Alexey Budankov wrote:
> 
> The patch set implements handling of 'start paused', 'resume' and 'pause'
> external control commands which can be provided for stat and record modes
> of the tool from an external controlling process. 'start paused' command
> can be used to postpone enabling of events in the beginning of a monitoring
> session. 'resume' and 'pause' commands can be used to enable and disable
> events correspondingly any time after the start of the session.
> 
> The 'start paused', resume and 'pause' external control commands can be
> used to focus measurement on specially selected time intervals of workload
> execution. Focused measurement reduces tool intrusion and influence on
> workload behavior, reduces distortion and amount of collected and stored
> data, mitigates data accuracy loss because measurement and data capturing
> happen only during intervals of interest.
> 
> A controlling process can be a bash shell script [1], native executable or
> any other language program that can directly work with file descriptors,
> e.g. pipes [2], and spawn a process, specially the tool one.
> 
> -D,--delay <val> option is extended with -1 value to skip events enabling
> in the beginning of a monitoring session ('start paused' command). --ctl-fd
> and --ctl-fd-ack command line options are introduced to provide the tool
> with a pair of file descriptors to listen to 'resume' and 'pause' commands
> and reply to an external controlling process on the completion of received
> commands processing.
> 
> The tool reads two byte control command message from ctl-fd descriptor,
> handles the command and optionally replies two bytes acknowledgement message
> to fd-ack descriptor, if it is specified on the command line. 'resume' command
> is recognized as 'r' character message and 'pause' command is recognized as
> 'p' character message both received from ctl-fd descriptor. Completion message
> is 'a''\n' and sent to fd-ack descriptor.
> 
> Bash script demonstrating simple use case follows:
> 
> #!/bin/bash
> 
> ctl_dir=/tmp/
> 
> ctl_fifo=${ctl_dir}perf_ctl.fifo
> test -p ${ctl_fifo} && unlink ${ctl_fifo}
> mkfifo ${ctl_fifo} && exec {ctl_fd}<>${ctl_fifo}
> 
> ctl_ack_fifo=${ctl_dir}perf_ctl_ack.fifo
> test -p ${ctl_ack_fifo} && unlink ${ctl_ack_fifo}
> mkfifo ${ctl_ack_fifo} && exec {ctl_fd_ack}<>${ctl_ack_fifo}
> 
> perf stat -D -1 -e cpu-cycles -a -I 1000                \
>           --ctl-fd ${ctl_fd} --ctl-fd-ack ${ctl_fd_ack} \
>           -- sleep 40 &

hi,
is fifo the best choice? do you need it for plug perf in somewhere?
what's your use case for this?

fifos seem complicated because you need to create 2 of them, would
unix socket be better maybe? and do we really need that ack fd?

also you could pass just path and perf could create fifos from them

  # perf stat --control-fifo /tmp/...

or to get really creazy, we could add option that would make perf
to listen on socket or whatever and we would control it via another
perf command ;-)

  # perf stat --control ....
  control socket: /tmp/xxx

  # perf stat control -s /tmp/xxx disable
  # perf stat control -s /tmp/xxx  enable

but ATM I can't see too much use for this, so would be great to
know your usecase ;-)

thanks,
jirka

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