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SubjectRe: [Patch][RFC] Disabling per-tgid stats on task exit in taskstats
Paul Jackson wrote:

>Shailabh wrote:
>
>
>>Sends a separate "registration" message with cpumask to listen to.
>>Kernel stores (real) pid and cpumask.
>>
>>
>
>Question:
>=========
>
>Ah - good.
>
>So this means that I could configure a system with a fork/exit
>intensive, performance critical job on some dedicated CPUs, and be able
>to collect taskstat data from tasks exiting on the -other- CPUS, while
>avoiding collecting data from this special job, thus avoiding any
>taskstat collection performance impact on said job.
>
>If I'm understanding this correctly, excellent.
>
>
Yes. If no one registers to listen on a particular CPU, data from tasks
exiting on that cpu is
not sent out at all.

>Caveat:
>=======
>
>Passing cpumasks across the kernel-user boundary can be tricky.
>
>Historically, Unix has a long tradition of boloxing up the passing
>of variable length data types across the kernel-user boundary.
>
>We've got perhaps a half dozen ways of getting these masks out of the
>kernel, and three ways of getting them (or the similar nodemasks) back
>into the kernel. The three ways being used in the sched_setaffinity
>system call, the mbind and set_mempolicy system calls, and the cpuset
>file system.
>
>All three of these ways have their controversial details:
> * The kernel cpumask mask size needed for sched_setaffinity calls is
> not trivially available to userland.
> * The nodemask bit size is off by one in the mbind and set_mempolicy
> calls.
> * The CPU and Node masks are ascii, not binary, in the cpuset calls.
>
>One option that might make sense for these task stat registrations
>would be to:
> 1) make the kernel/sched.c get_user_cpu_mask() routine generic,
> moving it to non-static lib/*.c code, and
> 2) provide a sensible way for user space to query the size of
> the kernel cpumask (and perhaps nodemask while you're at it.)
>
>Currently, the best way I know for user space to query the kernels
>cpumask and nodemask size is to examine the length of the ascii
>string values labeled "Cpus_allowed:" and "Mems_allowed:" in the file
>/proc/self/status. These ascii strings always require exactly nine
>ascii chars to express each 32 bits of kernel mask code, if you include
>in the count the trailing ',' comma or '\n' newline after each eight
>ascii character word.
>
>Probing /proc/self/status fields for these mask sizes is rather
>unobvious and indirect, and requires caching the result if you care at
>all about performance. Userland code in support of your taskstat
>facility might be better served by a more obvious way to size cpumasks.
>
>... unless of course you're inclined to pass cpumasks formatted as
> ascii strings, in which case speak up, as I'd be delighted to
> throw in my 2 cents on how to do that ;).
>
>
Thanks for the size info. I did hit it while coding this up.

So I chose to use the "cpulist" ascii format that has been helpfully
provided in include/linux/cpumask.h (by whom I wonder :-)

User specified the cpumask as an ascii string containing comma separated
cpu ranges.
Kernel parses the same and stores it as a cpumask_t after which we can
iterate over the
mask using standard helpers.

Since registration/deregistration is not a common operation, the
overhead of parsing
ascii strings should be acceptable and avoids the hassles of trying to
determine kernel cpumask size. I don't know if there are buffer overflow
issues in passing a string (though I'm using the
standard netlink way of passing it up using NLA_STRING).

Will post the patch shortly.

--Shailabh
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