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Message-ID: <20180620123208.GN2476@hirez.programming.kicks-ass.net>
Date: Wed, 20 Jun 2018 14:32:08 +0200
From: Peter Zijlstra <peterz@...radead.org>
To: Thomas Gleixner <tglx@...utronix.de>
Cc: Pavel Tatashin <pasha.tatashin@...cle.com>,
steven.sistare@...cle.com, daniel.m.jordan@...cle.com,
linux@...linux.org.uk, schwidefsky@...ibm.com,
heiko.carstens@...ibm.com, john.stultz@...aro.org,
sboyd@...eaurora.org, x86@...nel.org, linux-kernel@...r.kernel.org,
mingo@...hat.com, hpa@...or.com, douly.fnst@...fujitsu.com,
prarit@...hat.com, feng.tang@...el.com, pmladek@...e.com,
gnomes@...rguk.ukuu.org.uk
Subject: Re: [PATCH v10 7/7] x86/tsc: use tsc early
On Wed, Jun 20, 2018 at 12:42:40PM +0200, Thomas Gleixner wrote:
> On Wed, 20 Jun 2018, Peter Zijlstra wrote:
> > I'm still puzzled by the entire need for tsc_early_enabled and all that.
> > Esp. since both branches do the exact same thing:
> >
> > return cycles_2_ns(rdtsc());
>
> Right. But up to the point where the real sched_clock initialization can be
> done and the static keys can be flipped, there must be a way to
> conditinally use TSC depending on availablility and early initialization.
Ah, so we want to flip keys early, can be done, see below.
> You might argue, that we shouldn't care becasue the jiffies case is just
> the worst case fallback anyway. I wouldn't even disagree as those old
> machines which have TSC varying with the CPU frequency really should not
> matter anymore. Pavel might disagree of course.
You forgot (rightfully) that we even use TSC on those !constant
machines, we adjust the cycles_2_ns thing from the cpufreq notifiers.
The only case we should _ever_ use that jiffies callback is when TSC
really isn't there. Basically, if we kill notsc, we could make
native_sched_clock() := cycles_2_ns(rdtsc()) (for CONFIG_X86_TSC), the
end.
No static keys, nothing.
That said; flipping static keys early isn't hard. We should call
jump_label_init() early, because we want the entries sorted and the
key->entries link set. It will also replace the GENERIC_NOP5_ATOMIC
thing, which means we need to also do arch_init_ideal_nop() early, but
since that is pure CPUID based that should be doable.
And then something like the below could be used.
---
diff --git a/arch/x86/kernel/jump_label.c b/arch/x86/kernel/jump_label.c
index e56c95be2808..2dd8c5bdd87b 100644
--- a/arch/x86/kernel/jump_label.c
+++ b/arch/x86/kernel/jump_label.c
@@ -140,4 +140,38 @@ __init_or_module void arch_jump_label_transform_static(struct jump_entry *entry,
__jump_label_transform(entry, type, text_poke_early, 1);
}
+void jump_label_update_early(struct static_key *key, bool enable)
+{
+ struct jump_entry *entry, *stop = __stop___jump_table;
+
+ /*
+ * We need the table sorted and key->entries set up.
+ */
+ WARN_ON_ONCE(!static_key_initialized);
+
+ entry = static_key_entries(key);
+
+ /*
+ * Sanity check for early users, there had beter be a core kernel user.
+ */
+ if (!entry || !entry->code || !core_kernel_text(entry->code)) {
+ WARN_ON(1);
+ return;
+ }
+
+ for ( ; (entry < stop) && (jump_entry_key(entry) == key); entry++) {
+ enum jump_label_type type = enable ^ jump_entry_branch(entry);
+ __jump_label_transform(entry, type, text_poke_early, 0);
+ }
+
+ atomic_set_release(&key->enabled, !!enabled);
+}
+
+#else
+
+void jump_label_update_early(struct static_key *key, bool enable)
+{
+ atomic_set(&key->enabled, !!enabled);
+}
+
#endif
diff --git a/include/linux/jump_label.h b/include/linux/jump_label.h
index b46b541c67c4..cac61beca25f 100644
--- a/include/linux/jump_label.h
+++ b/include/linux/jump_label.h
@@ -79,6 +79,7 @@
#include <linux/types.h>
#include <linux/compiler.h>
+#include <linux/bug.h>
extern bool static_key_initialized;
@@ -110,6 +111,17 @@ struct static_key {
};
};
+#define JUMP_TYPE_FALSE 0UL
+#define JUMP_TYPE_TRUE 1UL
+#define JUMP_TYPE_LINKED 2UL
+#define JUMP_TYPE_MASK 3UL
+
+static inline struct jump_entry *static_key_entries(struct static_key *key)
+{
+ WARN_ON_ONCE(key->type & JUMP_TYPE_LINKED);
+ return (struct jump_entry *)(key->type & ~JUMP_TYPE_MASK);
+}
+
#else
struct static_key {
atomic_t enabled;
@@ -119,6 +131,17 @@ struct static_key {
#ifdef HAVE_JUMP_LABEL
#include <asm/jump_label.h>
+
+static inline struct static_key *jump_entry_key(struct jump_entry *entry)
+{
+ return (struct static_key *)((unsigned long)entry->key & ~1UL);
+}
+
+static inline bool jump_entry_branch(struct jump_entry *entry)
+{
+ return (unsigned long)entry->key & 1UL;
+}
+
#endif
#ifndef __ASSEMBLY__
@@ -132,11 +155,6 @@ struct module;
#ifdef HAVE_JUMP_LABEL
-#define JUMP_TYPE_FALSE 0UL
-#define JUMP_TYPE_TRUE 1UL
-#define JUMP_TYPE_LINKED 2UL
-#define JUMP_TYPE_MASK 3UL
-
static __always_inline bool static_key_false(struct static_key *key)
{
return arch_static_branch(key, false);
diff --git a/kernel/jump_label.c b/kernel/jump_label.c
index 01ebdf1f9f40..9710fa7582aa 100644
--- a/kernel/jump_label.c
+++ b/kernel/jump_label.c
@@ -295,12 +295,6 @@ void __weak __init_or_module arch_jump_label_transform_static(struct jump_entry
arch_jump_label_transform(entry, type);
}
-static inline struct jump_entry *static_key_entries(struct static_key *key)
-{
- WARN_ON_ONCE(key->type & JUMP_TYPE_LINKED);
- return (struct jump_entry *)(key->type & ~JUMP_TYPE_MASK);
-}
-
static inline bool static_key_type(struct static_key *key)
{
return key->type & JUMP_TYPE_TRUE;
@@ -321,16 +315,6 @@ static inline void static_key_set_linked(struct static_key *key)
key->type |= JUMP_TYPE_LINKED;
}
-static inline struct static_key *jump_entry_key(struct jump_entry *entry)
-{
- return (struct static_key *)((unsigned long)entry->key & ~1UL);
-}
-
-static bool jump_entry_branch(struct jump_entry *entry)
-{
- return (unsigned long)entry->key & 1UL;
-}
-
/***
* A 'struct static_key' uses a union such that it either points directly
* to a table of 'struct jump_entry' or to a linked list of modules which in
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