Re: [uml-devel] [PATCH] host context switch reduction

["Sven 'Darkman' Michels" <sven@darkman.de>]

Jeff Dike wrote:
> The patches below are from Laurent Vivier who didn't make them public.  They
> add a new feature to ptrace on the host which cuts down on the number of
> context switches needed for a UML system call, plus makes UML use it.
> 
> There's some interest in this, so I'm putting it out as-is.  I haven't played
> with it.

Well, we did now a bit. First, we need to change the patch a bit:
the patch for uml failed in arch/um/kernel/skas/process.c.
There was a call:
err = ptrace(PTRACE_SYSCALL, userspace_pid, 0, 0);
which is now:
err = ptrace(PTRACE_SYSCALL, pid, 0, 0);

so we changed all userspace_pid's in the patch to just pid. After that,
the uml kernel was ready to build which was done without any error.
But UML seem to go crazy a bit with the patch. In 2 of 3 times uml
crashed at startup:
Checking for host processor cmov support...Yes
Checking for host processor xmm support...No
Checking that ptrace can change system call numbers...OK
Checking syscall emulation patch for ptrace...OK
Lernel panic: check_ptrace : ptrace failed, errno = 3
In idle task - not syncing

Well, but since it sometimes starts we just start it as long as
it need to run ;) (for testing ok..)


> Laurent did some performance testing and found ~40% speedup on a getpid
> loop, and a 3.5% speedup on a kernel build.

Well, we just looked at the context switches. We started 3 UMLs which
were doing a make world for xfree. Without patches the number of
context switches per second was around 60k, with the patch it was around
40k - so it's noticeable, at least from the cs.

Sven







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