Re: [RFC][PATCH 2/4] tracing: Use pid bitmap instead of a pid array for set_event_pid

From: H. Peter Anvin
Date: Tue Apr 19 2016 - 14:58:01 EST

On April 19, 2016 10:19:47 AM PDT, Steven Rostedt <rostedt@xxxxxxxxxxx> wrote:
>On Tue, 19 Apr 2016 16:55:28 +0000 (UTC)
>Mathieu Desnoyers <mathieu.desnoyers@xxxxxxxxxxxx> wrote:
>> ----- On Apr 19, 2016, at 10:34 AM, rostedt rostedt@xxxxxxxxxxx
>> > From: Steven Rostedt <rostedt@xxxxxxxxxxx>
>> >
>> > In order to add the ability to let tasks that are filtered by the
>> > have their children also be traced on fork (and then not traced on
>> > convert the array into a pid bitmask. Most of the time the number
>of pids is
>> > only 32768 pids or a 4k bitmask, which is the same size as the
>default list
>> > currently is, and that list could grow if more pids are listed.
>> >
>> > This also greatly simplifies the code.
>> The maximum PID number can be increased with sysctl.
>> See "pid_max" in Documentation/sysctl/kernel.txt
>> What happens when you have a very large pid_max set ?
>I discussed this with HPA, and it appears that the pid_max max would
>require a bitmap of about 1/2 meg (the current default is 8k). This is
>also why I chose to keep the bitmap as vmalloc and not a continuous
>page allocation.
>> You say "most of the time" as if this was a fast-path vs a slow-path,
>> but it is not the case here.
>I meant "most of the time" as "default". Yes, you can make the pid_max
>really big, but in that case you better have enough memory in your
>system to handle that many threads. Thus a 1/2 meg used for tracking
>pids shouldn't be an issue.
>> This is a configuration option that can significantly hurt memory
>> in configurations using a large pid_max.
>No, it is created dynamically. If you never write anything into the
>set_event_pid file, then you have nothing to worry about, as nothing
>is allocated. It creates the array when a pid is added to the file, and
>only then. If it fails to allocate, the write will return -ENOMEM as
>Again, if you have a large pid_max your box had better have a lot of
>memory to begin with, because this array will be negligible compared to
>the memory required to handle large number of tasks.
>> FWIW, I implement a similar feature with a hash table in
>> I don't have the child process tracking though, which is a neat
>I originally had a complex hash algorithm because I too was worried
>about the size of pid_max and using a bitmap, but HPA convinced me it
>was the way to go.
>-- Steve

Also, I understand there is one of these bitmaps per ring buffer, and the ring buffer is in the tens of megabytes.
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