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

[Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca>]

* Paul Mackerras (paulus@samba.org) wrote:
> 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.
> 

Crazy ideas :

Could we do something along the lines of the thread local storage ?

Or could we map a per-thread page that would contradict this
"definition" ?

Or can we move down the beginning of the user-space thread stack of 4
bytes (it's already put at a random address anyway) and use these 32
bits to put our variable ? We don't care if userspace also modifies it;
the kernel would blindly increment it, so there would be no security
concerns involved.

Mathieu

-- 
Mathieu Desnoyers
Computer Engineering Ph.D. Student, Ecole Polytechnique de Montreal
OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F  BA06 3F25 A8FE 3BAE 9A68