Re: [RFC PATCH v3 00/16] Core scheduling v3

From: Matthew Garrett
Date: Tue Aug 27 2019 - 17:14:39 EST

Apple have provided a sysctl that allows applications to indicate that
specific threads should make use of core isolation while allowing
the rest of the system to make use of SMT, and browsers (Safari, Firefox
and Chrome, at least) are now making use of this. Trying to do something
similar using cgroups seems a bit awkward. Would something like this be
reasonable? Having spoken to the Chrome team, I believe that the
semantics we want are:

1) A thread to be able to indicate that it should not run on the same
core as anything not in posession of the same cookie
2) Descendents of that thread to (by default) have the same cookie
3) No other thread be able to obtain the same cookie
4) Threads not be able to rejoin the global group (ie, threads can
segregate themselves from their parent and peers, but can never rejoin
that group once segregated)

but don't know if that's what everyone else would want.

diff --git a/include/uapi/linux/prctl.h b/include/uapi/linux/prctl.h
index 094bb03b9cc2..5d411246d4d5 100644
--- a/include/uapi/linux/prctl.h
+++ b/include/uapi/linux/prctl.h
@@ -229,4 +229,5 @@ struct prctl_mm_map {
# define PR_PAC_APDBKEY (1UL << 3)
# define PR_PAC_APGAKEY (1UL << 4)

+#define PR_CORE_ISOLATE 55
#endif /* _LINUX_PRCTL_H */
diff --git a/kernel/sys.c b/kernel/sys.c
index 12df0e5434b8..a054cfcca511 100644
--- a/kernel/sys.c
+++ b/kernel/sys.c
@@ -2486,6 +2486,13 @@ SYSCALL_DEFINE5(prctl, int, option, unsigned long, arg2, unsigned long, arg3,
return -EINVAL;
error = PAC_RESET_KEYS(me, arg2);
+ current->core_cookie = (unsigned long)current;
+ result = -EINVAL;
+ break;
error = -EINVAL;

Matthew Garrett | mjg59@xxxxxxxxxxxxx