On Mon, Jul 21, 2014 at 05:32:55PM -0700, Davidlohr Bueso wrote: > On Mon, 2014-07-21 at 14:31 -0700, Andy Lutomirski wrote: > > On Mon, Jul 21, 2014 at 2:27 PM, Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote: > > > All this is predicated on the fact that syscalls are 'expensive'. > > > Weren't syscalls only 100s of cycles? All this bitmap mucking is far > > > more expensive due to cacheline misses, which due to the size of the > > > things is almost guaranteed. > > > > 120 - 300 cycles for me, unless tracing happens, and I'm working on > > reducing the incidence of tracing. > > fwiw here's what lmbench's lat_ctx says on my system . For 'accuracy', I > kept the runs short. > > http://www.stgolabs.net/lat_ctx.png Create a syscall without 'work body', so say something like the below. Context switches do quite a lot of work (unfortunately). diff --git a/arch/x86/syscalls/syscall_64.tbl b/arch/x86/syscalls/syscall_64.tbl index ec255a1646d2..daf27ec10e29 100644 --- a/arch/x86/syscalls/syscall_64.tbl +++ b/arch/x86/syscalls/syscall_64.tbl @@ -323,6 +323,7 @@ 314 common sched_setattr sys_sched_setattr 315 common sched_getattr sys_sched_getattr 316 common renameat2 sys_renameat2 +317 common nop sys_nop # # x32-specific system call numbers start at 512 to avoid cache impact diff --git a/kernel/sys.c b/kernel/sys.c index 66a751ebf9d9..1b86614bb551 100644 --- a/kernel/sys.c +++ b/kernel/sys.c @@ -2108,6 +2108,11 @@ SYSCALL_DEFINE1(sysinfo, struct sysinfo __user *, info) return 0; } +SYSCALL_DEFINE0(nop) +{ + return 0; +} + #ifdef CONFIG_COMPAT struct compat_sysinfo { s32 uptime; -- To unsubscribe from this list: send the line "unsubscribe linux-api" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
