how to implement platform specific per process parameter?
aik at ozlabs.ru
Mon Aug 6 15:08:28 EST 2012
There is a per core register on modern POWER processors (POWER6+) called
"DSCR": The DSCR (aka Data Stream Control Register) is supported on some
server PowerPC chips and allow some control over the prefetch of data
Its "per process" support has been added by
"powerpc: Per process DSCR + some fixes (try#4)"
The patch adds the DSCR value into "struct thread_struct" and
saves/restores it when the context is switched. Also, there is some policy
to initialize the register when a thread starts. So far so good.
Here is the problem.
Some customer(s) of this feature want an ability to change the DSCR value
of the already running process. I.e. we need some kernel interface which
would accept pid and the register value and change it.
There are 4 options:
1. [not a kernel interface] use ptrace to execute the register changing
command inside the specified pid. The next context switch saves the new
value in the thread_struct. Dirty hack.
2. Add a new syscall which would receive pid + register value and do the
job. A bit too much.
3. Add some hook in /proc filesystem but so far there were no platform
specific bits, right?
4. Implement a static node "/sys/devices/system/cpu/dscr_control". write()
would parse the input stream, call scanf("%d %x", &pid, &dscr) and do the job.
What is the correct approach? Thanks.
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