[tip: x86/fpu] x86/fpu: Check TIF_NEED_FPU_LOAD instead of PF_KTHREAD|PF_USER_WORKER in fpu__drop()

[tip-bot2 for Oleg Nesterov]

The following commit has been merged into the x86/fpu branch of tip:

Commit-ID:     016a2e6f8ae5ed544ba8fb2b6d78f64ddfd9d01b
Gitweb:        https://git.kernel.org/tip/016a2e6f8ae5ed544ba8fb2b6d78f64ddfd9d01b
Author:        Oleg Nesterov <oleg@xxxxxxxxxx>
AuthorDate:    Sat, 03 May 2025 16:38:56 +02:00
Committer:     Ingo Molnar <mingo@xxxxxxxxxx>
CommitterDate: Sun, 04 May 2025 10:29:25 +02:00

x86/fpu: Check TIF_NEED_FPU_LOAD instead of PF_KTHREAD|PF_USER_WORKER in fpu__drop()

PF_KTHREAD|PF_USER_WORKER tasks should never clear TIF_NEED_FPU_LOAD,
so the TIF_NEED_FPU_LOAD check should equally filter them out.

And this way an exiting userspace task can avoid the unnecessary "fwait"
if it does context_switch() at least once on its way to exit_thread().

Signed-off-by: Oleg Nesterov <oleg@xxxxxxxxxx>
Signed-off-by: Ingo Molnar <mingo@xxxxxxxxxx>
Cc: Chang S . Bae <chang.seok.bae@xxxxxxxxx>
Cc: H. Peter Anvin <hpa@xxxxxxxxx>
Cc: Andy Lutomirski <luto@xxxxxxxxxxxxxx>
Cc: Brian Gerst <brgerst@xxxxxxxxx>
Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Link: https://lore.kernel.org/r/20250503143856.GA9009@xxxxxxxxxx
---
 arch/x86/kernel/fpu/core.c | 3 +--
 1 file changed, 1 insertion(+), 2 deletions(-)

diff --git a/arch/x86/kernel/fpu/core.c b/arch/x86/kernel/fpu/core.c
index 8d67443..fa13129 100644
--- a/arch/x86/kernel/fpu/core.c
+++ b/arch/x86/kernel/fpu/core.c
@@ -693,8 +693,7 @@ void fpu__drop(struct task_struct *tsk)
 {
 	struct fpu *fpu;
 
-	/* PF_KTHREAD tasks do not use the FPU context area: */
-	if (tsk->flags & (PF_KTHREAD | PF_USER_WORKER))
+	if (test_tsk_thread_flag(tsk, TIF_NEED_FPU_LOAD))
 		return;
 
 	fpu = x86_task_fpu(tsk);