[RFC PATCH] sched: Remove set_task_state()
mark.rutland at arm.com
Tue Jan 3 02:04:53 PST 2017
On Fri, Dec 30, 2016 at 10:17:53AM -0800, Davidlohr Bueso wrote:
> Secondly for a higher overview, an unlink microbenchmark was used,
> which pounds on a single file with open, close,unlink combos with
> increasing thread counts (up to 4x ncpus). While the workload is
> quite unrealistic, it does contend a lot on the inode mutex or now
> rwsem. With the archs I had access to, the differences are as follows:
> == 1. arm64 ==
> 0000000000002784 <set_task_state>:
> 2784: f9000c1f str xzr, [x0,#24]
> 0000000000002790 <set_current_state>:
> 2790: d5384100 mrs x0, sp_el0
> 2794: f9000c1f str xzr, [x0,#24]
> Avg runtime set_task_state(): 2648 msecs
> Avg runtime set_current_state(): 2686 msecs
> Unsurprisingly, the big looser is arm64, due to the masking of sp_el0.
> otoh, x86-64 (known to be fast for get_current()/this_cpu_read_stable()
> caching) and ppc64 (with paca) show similar improvements in the unlink
> microbenches. x86's write latencies delta is similar to the opposite of
> arm64: 50ms vs -40ms, respectively. The small delta for ppc64 (2ms), does
> not represent the gains on the unlink runs. In the case of x86, there was
> a decent amount of variation in the latency runs, but always within a 20
> to 50ms increase), ppc was more constant.
> So, do we want to get rid of the interface (and improve performance on
> other archs) at the expense of arm64? Can arm64 do better?
We can defineitely do better; the asm constraints in read_sysreg() are
overly pessimistic for get_current().
Does the below help?
diff --git a/arch/arm64/include/asm/current.h b/arch/arm64/include/asm/current.h
index f2bcbe2..c9ba5ac 100644
@@ -11,7 +11,11 @@
static __always_inline struct task_struct *get_current(void)
- return (struct task_struct *)read_sysreg(sp_el0);
+ struct task_struct *tsk;
+ asm ("mrs %0, sp_el0" : "=r" (tsk));
+ return tsk;
#define current get_current()
More information about the linux-arm-kernel