Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752753AbbDCDjN (ORCPT ); Thu, 2 Apr 2015 23:39:13 -0400 Received: from g4t3427.houston.hp.com ([15.201.208.55]:48526 "EHLO g4t3427.houston.hp.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751083AbbDCDjL (ORCPT ); Thu, 2 Apr 2015 23:39:11 -0400 Message-ID: <551E0B58.5070005@hp.com> Date: Thu, 02 Apr 2015 23:39:04 -0400 From: Waiman Long User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:10.0.12) Gecko/20130109 Thunderbird/10.0.12 MIME-Version: 1.0 To: Peter Zijlstra CC: tglx@linutronix.de, mingo@redhat.com, hpa@zytor.com, paolo.bonzini@gmail.com, konrad.wilk@oracle.com, boris.ostrovsky@oracle.com, paulmck@linux.vnet.ibm.com, riel@redhat.com, torvalds@linux-foundation.org, raghavendra.kt@linux.vnet.ibm.com, david.vrabel@citrix.com, oleg@redhat.com, scott.norton@hp.com, doug.hatch@hp.com, linux-arch@vger.kernel.org, x86@kernel.org, linux-kernel@vger.kernel.org, virtualization@lists.linux-foundation.org, xen-devel@lists.xenproject.org, kvm@vger.kernel.org, luto@amacapital.net Subject: Re: [PATCH 8/9] qspinlock: Generic paravirt support References: <551C1ACE.4090408@hp.com> <20150401171223.GO23123@twins.programming.kicks-ass.net> <20150401174239.GO24151@twins.programming.kicks-ass.net> <20150401181744.GE32047@worktop.ger.corp.intel.com> <551C3EF5.6090809@hp.com> <20150401184858.GA9791@dyad.arnhem.chello.nl> <551C4E02.8030806@hp.com> <20150401210317.GZ27490@worktop.programming.kicks-ass.net> <551D6E2E.1080801@hp.com> <20150402172057.GA27490@worktop.programming.kicks-ass.net> <20150402194834.GF32047@worktop.ger.corp.intel.com> In-Reply-To: <20150402194834.GF32047@worktop.ger.corp.intel.com> Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 6622 Lines: 235 On 04/02/2015 03:48 PM, Peter Zijlstra wrote: > On Thu, Apr 02, 2015 at 07:20:57PM +0200, Peter Zijlstra wrote: >> pv_wait_head(): >> >> pv_hash() >> /* MB as per cmpxchg */ >> cmpxchg(&l->locked, _Q_LOCKED_VAL, _Q_SLOW_VAL); >> >> VS >> >> __pv_queue_spin_unlock(): >> >> if (xchg(&l->locked, 0) != _Q_SLOW_VAL) >> return; >> >> /* MB as per xchg */ >> pv_hash_find(lock); >> >> > > Something like so.. compile tested only. > > I took out the LFSR because that was likely over engineering from my > side :-) > > --- a/kernel/locking/qspinlock_paravirt.h > +++ b/kernel/locking/qspinlock_paravirt.h > @@ -2,6 +2,8 @@ > #error "do not include this file" > #endif > > +#include > + > /* > * Implement paravirt qspinlocks; the general idea is to halt the vcpus instead > * of spinning them. > @@ -107,7 +109,84 @@ 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 linear 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 adressing > + * 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 > + * > + */ > + > +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) > + > +#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); > +} > + > +#define for_each_hash_bucket(hb, off, hash) \ > + for (hash = hash_align(hash), off = 0, hb =&__pv_lock_hash[hash + off];\ > + off< PV_LOCK_HASH_SIZE; \ > + off++, hb =&__pv_lock_hash[(hash + off) % PV_LOCK_HASH_SIZE]) > + > +static struct pv_hash_bucket *pv_hash_insert(struct qspinlock *lock) > +{ > + u32 offset, hash = hash_ptr(lock, PV_LOCK_HASH_BITS); > + struct pv_hash_bucket *hb; > + > + for_each_hash_bucket(hb, offset, hash) { > + if (!cmpxchg(&hb->lock, NULL, lock)) { > + WRITE_ONCE(hb->cpu, smp_processor_id()); > + return hb; > + } > + } > + > + /* > + * Hard assumes there is an empty bucket somewhere. > + */ > + BUG(); > +} > + > +static struct pv_hash_bucket *pv_hash_find(struct qspinlock *lock) > +{ > + u32 offset, hash = hash_ptr(lock, PV_LOCK_HASH_BITS); > + struct pv_hash_bucket *hb; > + > + for_each_hash_bucket(hb, offset, hash) { > + if (READ_ONCE(hb->lock) == lock) > + return hb; > + } > + > + /* > + * Hard assumes we _WILL_ find a match. > + */ > + BUG(); > +} > > /* > * Wait for l->locked to become clear; halt the vcpu after a short spin. > @@ -116,7 +195,9 @@ static DEFINE_PER_CPU(struct qspinlock * > static void pv_wait_head(struct qspinlock *lock) > { > struct __qspinlock *l = (void *)lock; > + struct pv_hash_bucket *hb = NULL; > int loop; > + u8 o; > > for (;;) { > for (loop = SPIN_THRESHOLD; loop; loop--) { > @@ -126,29 +207,47 @@ static void pv_wait_head(struct qspinloc > cpu_relax(); > } > > - this_cpu_write(__pv_lock_wait, lock); > - /* > - * __pv_lock_wait must be set before setting _Q_SLOW_VAL > - * > - * [S] __pv_lock_wait = lock [RmW] l = l->locked = 0 > - * MB MB > - * [S] l->locked = _Q_SLOW_VAL [L] __pv_lock_wait > - * > - * Matches the xchg() in pv_queue_spin_unlock(). > - */ > - if (!cmpxchg(&l->locked, _Q_LOCKED_VAL, _Q_SLOW_VAL)) > - goto done; > + if (!hb) { > + hb = pv_hash_insert(lock); > + /* > + * hb must be set before setting _Q_SLOW_VAL > + * > + * [S] hb<- lock [RmW] l = l->locked = 0 > + * MB MB > + * [RmW] l->locked ?= _Q_SLOW_VAL [L] hb > + * > + * Matches the xchg() in pv_queue_spin_unlock(). > + */ > + o = cmpxchg(&l->locked, _Q_LOCKED_VAL, _Q_SLOW_VAL); > + if (!o) { > + /* > + * The lock got unlocked before we could set > + * _Q_SLOW_VAL, we must unhash ourselves. > + */ > + WRITE_ONCE(hb->lock, NULL); > + goto done; > + } > + BUG_ON(o != _Q_LOCKED_VAL); > + /* > + * At this point _Q_SLOW_VAL is visible and the unlock > + * will do the lookup. The lookup hard relies on the > + * entry being visible -- which it should be. Unlock > + * will unhash for us. > + */ > + } > > pv_wait(&l->locked, _Q_SLOW_VAL); > + /* > + * We can get spurious wakeups from interrupts, cycle back. > + */ > } > done: > - this_cpu_write(__pv_lock_wait, NULL); > - > /* > * Lock is unlocked now; the caller will acquire it without waiting. > * As with pv_wait_node() we rely on the caller to do a load-acquire > * for us. > */ > + return; > } > > /* > @@ -158,20 +257,20 @@ static void pv_wait_head(struct qspinloc > void __pv_queue_spin_unlock(struct qspinlock *lock) > { > struct __qspinlock *l = (void *)lock; > - int cpu; > + struct pv_hash_bucket *hb; > > if (xchg(&l->locked, 0) != _Q_SLOW_VAL) > return; > > /* > * At this point the memory pointed at by lock can be freed/reused, > - * however we can still use the pointer value to search in our cpu > - * array. > + * however we can still use the pointer value to search in our hash > + * table. > * > - * XXX: get rid of this loop > + * Also, if we observe _Q_SLOW_VAL we _must_ now observe 'our' hash > + * bucket. See pv_wait_head(). > */ > - for_each_possible_cpu(cpu) { > - if (per_cpu(__pv_lock_wait, cpu) == lock) > - pv_kick(cpu); > - } > + hb = pv_hash_find(lock); > + pv_kick(hb->cpu); > + WRITE_ONCE(hb->lock, NULL); /* unhash */ > } Thanks for the code. I am working on my own version, too. Will need to run some tests to verify the correctness of the code. Hopefully I have something for you to review early next week. Cheers, Longman -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/