* Mathieu Desnoyers <mathieu.desnoyers@xxxxxxxxxx> wrote: > * Ingo Molnar (mingo@xxxxxxx) wrote: > > > > * Mathieu Desnoyers <mathieu.desnoyers@xxxxxxxxxx> wrote: > > > > > In the category "crazy ideas one should never express out loud", I > > > could add the following. We could choose to save/restore the cr2 > > > register on the local stack at every interrupt entry/exit, and > > > therefore allow the page fault handler to execute with interrupts > > > enabled. > > > > > > I have not benchmarked the interrupt disabling overhead of the > > > page fault handler handled by starting an interrupt-gated handler > > > rather than trap-gated handler, but cli/sti instructions are known > > > to take quite a few cycles on some architectures. e.g. 131 cycles > > > for the pair on P4, 23 cycles on AMD Athlon X2 64, 43 cycles on > > > Intel Core2. > > > > The cost on Nehalem (1 billion local_irq_save()+restore() pairs): > > > > aldebaran:~> perf stat --repeat 5 ./prctl 0 0 > > > > Performance counter stats for './prctl 0 0' (5 runs): > > > > 10950.813461 task-clock-msecs # 0.997 CPUs ( +- 1.594% ) > > 3 context-switches # 0.000 M/sec ( +- 0.000% ) > > 1 CPU-migrations # 0.000 M/sec ( +- 0.000% ) > > 145 page-faults # 0.000 M/sec ( +- 0.000% ) > > 33946294720 cycles # 3099.888 M/sec ( +- 1.132% ) > > 8030365827 instructions # 0.237 IPC ( +- 0.006% ) > > 100933 cache-references # 0.009 M/sec ( +- 12.568% ) > > 27250 cache-misses # 0.002 M/sec ( +- 3.897% ) > > > > 10.985768499 seconds time elapsed. > > > > That's 33.9 cycles per iteration, with a 1.1% confidence factor. > > > > Annotation gives this result: > > > > 2.24 : ffffffff810535e5: 9c pushfq > > 8.58 : ffffffff810535e6: 58 pop %rax > > 10.99 : ffffffff810535e7: fa cli > > 20.38 : ffffffff810535e8: 50 push %rax > > 0.00 : ffffffff810535e9: 9d popfq > > 46.71 : ffffffff810535ea: ff c6 inc %esi > > 0.42 : ffffffff810535ec: 3b 35 72 31 76 00 cmp 0x763172(%rip),%e > > 10.69 : ffffffff810535f2: 7c f1 jl ffffffff810535e5 > > 0.00 : ffffffff810535f4: e9 7c 01 00 00 jmpq ffffffff81053775 > > > > i.e. pushfq+cli is roughly 42.19% or 14 cycles, the popfq is 46.71 > > or 16 cycles. So the combo cost is 30 cycles, +- 1 cycle. > > > > (Actual effective cost in a real critical section can be better than > > this, dependent on surrounding instructions.) > > > > It got quite a bit faster than Core2 - but still not as fast as AMD. > > > > Ingo > > Interesting, but in our specific case, what would be even more > interesting to know is how many trap gates/s vs interrupt gates/s > can be called. This would allow us to see if it's worth trying to > make the page fault handler interrupt-safe by mean of atomicity > and context save/restore by interrupt handlers (which would let us > run the PF handler with interrupts enabled). See the numbers in the other mail: about 33 million pagefaults happen in a typical kernel build - that's ~400K/sec - and that is not a particularly really pagefault-heavy workload. OTOH, interrupt gates, if above 10K/second, do get noticed and get reduced. Above 100K/sec combined they are really painful. In practice, a combo limit of 10K is healthy. So there's about an order of magnitude difference in the frequency of IRQs versus the frequency of pagefaults. In the worst-case, we have 10K irqs/sec and almost zero pagefaults - every 10 cycles overhead in irq entry+exit cost causes a 0.003% total slowdown. So i'd say that it's pretty safe to say that the shuffling of overhead from the pagefault path into the irq path, even if it's a zero-sum game as per cycles, is an overall win - or even in the worst-case, a negligible overhead. Syscalls are even more critical: it's easy to have a 'good' workload with millions of syscalls per second - so getting even a single cycle off the syscall entry+exit path is worth the pain. Ingo -- To unsubscribe from this list: send the line "unsubscribe linux-tip-commits" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html