Re: [RFC PATCH 16/22 -v2] add get_monotonic_cycles

Paul Mackerras
2008-01-17T03:21:36Z

Mathieu Desnoyers writes:

> Sorry for self-reply, but I thought, in the past, of a way to make this
> possible.
>
> It would imply the creation of a new vsyscall : vgetschedperiod
>
> It would read a counter that would increment each time the thread is
> scheduled out (or in). It would be a per thread counter

It's very hard to do a per-thread counter in the VDSO, since threads
in the same process see the same memory, by definition. You'd have to
have an array of counters and have some way for each thread to know
which entry to read. Also you'd have to find space for tens or
hundreds of thousands of counters, since there can be that many
threads in a process sometimes.

Paul.