From e07e23e1fd3000289fc7ccc6c71879070d3b19e0 Mon Sep 17 00:00:00 2001 From: Arjan van de Ven Date: Tue, 26 Sep 2006 10:52:36 +0200 Subject: [PATCH] non lazy "sleazy" fpu implementation Right now the kernel on x86-64 has a 100% lazy fpu behavior: after *every* context switch a trap is taken for the first FPU use to restore the FPU context lazily. This is of course great for applications that have very sporadic or no FPU use (since then you avoid doing the expensive save/restore all the time). However for very frequent FPU users... you take an extra trap every context switch. The patch below adds a simple heuristic to this code: After 5 consecutive context switches of FPU use, the lazy behavior is disabled and the context gets restored every context switch. If the app indeed uses the FPU, the trap is avoided. (the chance of the 6th time slice using FPU after the previous 5 having done so are quite high obviously). After 256 switches, this is reset and lazy behavior is returned (until there are 5 consecutive ones again). The reason for this is to give apps that do longer bursts of FPU use still the lazy behavior back after some time. [akpm@osdl.org: place new task_struct field next to jit_keyring to save space] Signed-off-by: Arjan van de Ven Signed-off-by: Andi Kleen Cc: Andi Kleen Signed-off-by: Andrew Morton --- arch/x86_64/kernel/process.c | 10 ++++++++++ 1 file changed, 10 insertions(+) (limited to 'arch/x86_64/kernel/process.c') diff --git a/arch/x86_64/kernel/process.c b/arch/x86_64/kernel/process.c index 6fbd19564e4e..9e9a70e50c72 100644 --- a/arch/x86_64/kernel/process.c +++ b/arch/x86_64/kernel/process.c @@ -552,6 +552,10 @@ __switch_to(struct task_struct *prev_p, struct task_struct *next_p) int cpu = smp_processor_id(); struct tss_struct *tss = &per_cpu(init_tss, cpu); + /* we're going to use this soon, after a few expensive things */ + if (next_p->fpu_counter>5) + prefetch(&next->i387.fxsave); + /* * Reload esp0, LDT and the page table pointer: */ @@ -629,6 +633,12 @@ __switch_to(struct task_struct *prev_p, struct task_struct *next_p) || test_tsk_thread_flag(prev_p, TIF_IO_BITMAP)) __switch_to_xtra(prev_p, next_p, tss); + /* If the task has used fpu the last 5 timeslices, just do a full + * restore of the math state immediately to avoid the trap; the + * chances of needing FPU soon are obviously high now + */ + if (next_p->fpu_counter>5) + math_state_restore(); return prev_p; } -- cgit v1.2.2