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Date: Thu, 19 Mar 2015 19:25:17 -0400
From: Waiman Long <waiman.long@...com>
To: Peter Zijlstra <peterz@...radead.org>
CC: tglx@...utronix.de, mingo@...hat.com, hpa@...or.com,
paolo.bonzini@...il.com, konrad.wilk@...cle.com,
boris.ostrovsky@...cle.com, paulmck@...ux.vnet.ibm.com,
riel@...hat.com, torvalds@...ux-foundation.org,
raghavendra.kt@...ux.vnet.ibm.com, david.vrabel@...rix.com,
oleg@...hat.com, scott.norton@...com, doug.hatch@...com,
linux-arch@...r.kernel.org, x86@...nel.org,
linux-kernel@...r.kernel.org,
virtualization@...ts.linux-foundation.org,
xen-devel@...ts.xenproject.org, kvm@...r.kernel.org,
luto@...capital.net
Subject: Re: [PATCH 8/9] qspinlock: Generic paravirt support
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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