Messages in this thread | | | Date | Thu, 4 Mar 2010 21:23:04 +0100 | From | Ingo Molnar <> | Subject | Re: [PATCH -tip] introduce sys_membarrier(): process-wide memory barrier (v9) |
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* Linus Torvalds <torvalds@linux-foundation.org> wrote:
> > On Thu, 4 Mar 2010, Ingo Molnar wrote: > > > > - SA_NOFPU: on x86 to skip the FPU/SSE save/restore, for such fast in/out special > > purpose signal handlers? (can whip up a quick patch for you if you want) > > I'd love to do this, but it's wrong. > > It's too damn easy to use the FPU by mistake in user land, without ever > being aware of it. memset()/memcpy are obvious potential users SSE, but they > might be called in non-obvious ways implicitly by the compiler (ie structure > copy and setup). > > And modern glibc ends up using SSE4 even for things like strstr and strlen, > so it really is creeping into all kinds of trivial helper functions that > might not be obvious. So SA_NOFPU is a lovely idea, but it's also an idea > that sucks rotten eggs in practice, with quite possibly the same _binary_ > working or not working depending on what kind of CPU and what shared library > it happens to be using. > > Too damn fragile, in other words. > > (Now, if it's accompanied by the kernel actually _testing_ that there is no > FPU activity, by setting the TS flag and checking at fault time and causing > a SIGFPE, then that would be better. At least you'd get a nice clear signal > rather than random FPU state corruption. But you're still in the situation > that now the binary might work on some machines and setups, and not on > others.
Perhaps NOFPU could do lazy context saving: clear the TS flag and only save the FPU state if it's actually used by the signal handler?
This turns it into a 'hint', not into an FPU state corruption issue.
Clearing/enabling FPU instructions is still faster than a full-blown FPU context save/restore.
Careful and lightweight signal handlers (like a GC scheme would likely be) would thus be faster. In the worst-case it incures an extra trap and a (measurable/profilable) slowdown.
In any case this would be a secondary optimization - the biggest difference i'd expect from the 'dont wake up the world' logic:
> > - SA_RUNNING: a way to signal only running threads - as a way for user-space > > based concurrency control mechanisms to deschedule running threads (or, like > > in your case, to implement barrier / garbage collection schemes). > > Hmm. This sounds less fundamentally broken, but at the same time also _way_ > more invasive in the signal handling layer. It's already one of our more > "exciting" layers out there.
Yeah, definitely. But i still tend to think it should be actively tried, at which point we can still say 'yuck this cannot work, lets go for the sys_membarrier() solution'.
Ingo
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