On 03/19/2015 08:25 AM, Peter Zijlstra wrote:
On Thu, Mar 19, 2015 at 11:12:42AM +0100, Peter Zijlstra wrote:
So I was now thinking of hashing the lock pointer; let me go and quickly
put something together.
A little something like so; ideally we'd allocate the hashtable since
NR_CPUS is kinda bloated, but it shows the idea I think.
And while this has loops in (the rehashing thing) their fwd progress
does not depend on other CPUs.
And I suspect that for the typical lock contention scenarios its
unlikely we ever really get into long rehashing chains.
---
include/linux/lfsr.h | 49 ++++++++++++
kernel/locking/qspinlock_paravirt.h | 143 ++++++++++++++++++++++++++++++++----
2 files changed, 178 insertions(+), 14 deletions(-)
This is a much better alternative.
--- /dev/null
+++ b/include/linux/lfsr.h
@@ -0,0 +1,49 @@
+#ifndef _LINUX_LFSR_H
+#define _LINUX_LFSR_H
+
+/*
+ * Simple Binary Galois Linear Feedback Shift Register
+ *
+ * http://en.wikipedia.org/wiki/Linear_feedback_shift_register
+ *
+ */
+
+extern void __lfsr_needs_more_taps(void);
+
+static __always_inline u32 lfsr_taps(int bits)
+{
+ if (bits == 1) return 0x0001;
+ if (bits == 2) return 0x0001;
+ if (bits == 3) return 0x0003;
+ if (bits == 4) return 0x0009;
+ if (bits == 5) return 0x0012;
+ if (bits == 6) return 0x0021;
+ if (bits == 7) return 0x0041;
+ if (bits == 8) return 0x008E;
+ if (bits == 9) return 0x0108;
+ if (bits == 10) return 0x0204;
+ if (bits == 11) return 0x0402;
+ if (bits == 12) return 0x0829;
+ if (bits == 13) return 0x100D;
+ if (bits == 14) return 0x2015;
+
+ /*
+ * For more taps see:
+ * http://users.ece.cmu.edu/~koopman/lfsr/index.html
+ */
+ __lfsr_needs_more_taps();
+
+ return 0;
+}
+
+static inline u32 lfsr(u32 val, int bits)
+{
+ u32 bit = val& 1;
+
+ val>>= 1;
+ if (bit)
+ val ^= lfsr_taps(bits);
+ return val;
+}
+
+#endif /* _LINUX_LFSR_H */
--- a/kernel/locking/qspinlock_paravirt.h
+++ b/kernel/locking/qspinlock_paravirt.h
@@ -2,6 +2,9 @@
#error "do not include this file"
#endif
+#include<linux/hash.h>
+#include<linux/lfsr.h>
+
/*
* Implement paravirt qspinlocks; the general idea is to halt the vcpus instead
* of spinning them.
@@ -107,7 +110,120 @@ static void pv_kick_node(struct mcs_spin
pv_kick(pn->cpu);
}
-static DEFINE_PER_CPU(struct qspinlock *, __pv_lock_wait);
+/*
+ * Hash table using open addressing with an LFSR probe sequence.
+ *
+ * Since we should not be holding locks from NMI context (very rare indeed) the
+ * max load factor is 0.75, which is around the point where open addressing
+ * breaks down.
+ *
+ * Instead of probing just the immediate bucket we probe all buckets in the
+ * same cacheline.
+ *
+ * http://en.wikipedia.org/wiki/Hash_table#Open_addressing
+ *
+ */
+
+#define HB_RESERVED ((struct qspinlock *)1)
+
+struct pv_hash_bucket {
+ struct qspinlock *lock;
+ int cpu;
+};
+
+/*
+ * XXX dynamic allocate using nr_cpu_ids instead...
+ */
+#define PV_LOCK_HASH_BITS (2 + NR_CPUS_BITS)
+
As said here, we should make it dynamically allocated depending on
num_possible_cpus().
+#if PV_LOCK_HASH_BITS< 6
+#undef PV_LOCK_HASH_BITS
+#define PB_LOCK_HASH_BITS 6
+#endif
+
+#define PV_LOCK_HASH_SIZE (1<< PV_LOCK_HASH_BITS)
+
+static struct pv_hash_bucket __pv_lock_hash[PV_LOCK_HASH_SIZE] ____cacheline_aligned;
+
+#define PV_HB_PER_LINE (SMP_CACHE_BYTES / sizeof(struct pv_hash_bucket))
+
+static inline u32 hash_align(u32 hash)
+{
+ return hash& ~(PV_HB_PER_LINE - 1);
+}
+
+static struct qspinlock **pv_hash(struct qspinlock *lock)
+{
+ u32 hash = hash_ptr(lock, PV_LOCK_HASH_BITS);
+ struct pv_hash_bucket *hb, *end;
+
+ if (!hash)
+ hash = 1;
+
+ hb =&__pv_lock_hash[hash_align(hash)];
+ for (;;) {
+ for (end = hb + PV_HB_PER_LINE; hb< end; hb++) {
+ if (cmpxchg(&hb->lock, NULL, HB_RESERVED)) {
+ WRITE_ONCE(hb->cpu, smp_processor_id());
+ /*
+ * Since we must read lock first and cpu
+ * second, we must write cpu first and lock
+ * second, therefore use HB_RESERVE to mark an
+ * entry in use before writing the values.
+ *
+ * This can cause hb_hash_find() to not find a
+ * cpu even though _Q_SLOW_VAL, this is not a
+ * problem since we re-check l->locked before
+ * going to sleep and the unlock will have
+ * cleared l->locked already.
+ */
+ smp_wmb(); /* matches rmb from pv_hash_find */
+ WRITE_ONCE(hb->lock, lock);
+ goto done;
+ }
+ }
+
+ hash = lfsr(hash, PV_LOCK_HASH_BITS);
+ hb =&__pv_lock_hash[hash_align(hash)];
+ }
+
+done:
+ return&hb->lock;
+}
+
+static int pv_hash_find(struct qspinlock *lock)
+{
+ u64 hash = hash_ptr(lock, PV_LOCK_HASH_BITS);
+ struct pv_hash_bucket *hb, *end;
+ int cpu = -1;
+
+ if (!hash)
+ hash = 1;
+
+ hb =&__pv_lock_hash[hash_align(hash)];
+ for (;;) {
+ for (end = hb + PV_HB_PER_LINE; hb< end; hb++) {
+ struct qspinlock *l = READ_ONCE(hb->lock);
+
+ /*
+ * If we hit an unused bucket, there is no match.
+ */
+ if (!l)
+ goto done;
+
+ if (l == lock) {
+ smp_rmb(); /* matches wmb from pv_hash() */
+ cpu = READ_ONCE(hb->cpu);
+ goto done;
+ }
+ }
+
+ hash = lfsr(hash, PV_LOCK_HASH_BITS);
+ hb =&__pv_lock_hash[hash_align(hash)];
+ }
+done:
+ return cpu;
+}
We should probably abstract out the pv_hash and pv_hash_find into
generic functions that can be put into header like hash.h instead of
doing it locally here.
-Longman
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