Messages in this thread | | | Date | Mon, 20 Feb 2012 20:50:22 -0500 | Subject | Re: [PATCH v2 3/3] i387: support lazy restore of FPU state | From | Josh Boyer <> |
| |
On Mon, Feb 20, 2012 at 2:48 PM, Linus Torvalds <torvalds@linux-foundation.org> wrote: > > From: Linus Torvalds <torvalds@linux-foundation.org> > Date: Sun, 19 Feb 2012 13:27:00 -0800 > Subject: [PATCH v2 3/3] i387: support lazy restore of FPU state > > This makes us recognize when we try to restore FPU state that matches > what we already have in the FPU on this CPU, and avoids the restore > entirely if so. > > To do this, we add two new data fields: > > - a percpu 'fpu_owner_task' variable that gets written any time we > update the "has_fpu" field, and thus acts as a kind of back-pointer > to the task that owns the CPU. The exception is when we save the FPU > state as part of a context switch - if the save can keep the FPU > state around, we leave the 'fpu_owner_task' variable pointing at the > task whose FP state still remains on the CPU. > > - a per-thread 'last_cpu' field, that indicates which CPU that thread > used its FPU on last. We update this on every context switch > (writing an invalid CPU number if the last context switch didn't > leave the FPU in a lazily usable state), so we know that *that* > thread has done nothing else with the FPU since. > > These two fields together can be used when next switching back to the > task to see if the CPU still matches: if 'fpu_owner_task' matches the > task we are switching to, we know that no other task (or kernel FPU > usage) touched the FPU on this CPU in the meantime, and if the current > CPU number matches the 'last_cpu' field, we know that this thread did no > other FP work on any other CPU, so the FPU state on the CPU must match > what was saved on last context switch. > > In that case, we can avoid the 'f[x]rstor' entirely, and just clear the > CR0.TS bit. > > Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
I haven't tried really figuring this out yet, but building the Fedora kernel on x86_64 with your latest tree results in:
ERROR: "fpu_owner_task" [lib/raid6/raid6_pq.ko] undefined! ERROR: "fpu_owner_task" [arch/x86/kvm/kvm.ko] undefined! ERROR: "fpu_owner_task" [arch/x86/crypto/sha1-ssse3.ko] undefined! ERROR: "fpu_owner_task" [arch/x86/crypto/serpent-sse2-x86_64.ko] undefined! ERROR: "fpu_owner_task" [arch/x86/crypto/ghash-clmulni-intel.ko] undefined! make[1]: *** [__modpost] Error 1 make: *** [modules] Error 2 + exit 1
Since this patch went in as 7e16838d94b566a1, I'm guessing it's at least related.
I'm building again with more verbose output but I thought I'd send this out quickly.
josh -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
| |