Waiman Long
2014-Oct-24 20:53 UTC
[PATCH v12 09/11] pvqspinlock, x86: Add para-virtualization support
On 10/24/2014 04:47 AM, Peter Zijlstra wrote:> On Thu, Oct 16, 2014 at 02:10:38PM -0400, Waiman Long wrote: >> +static inline void pv_init_node(struct mcs_spinlock *node) >> +{ >> + struct pv_qnode *pn = (struct pv_qnode *)node; >> + >> + BUILD_BUG_ON(sizeof(struct pv_qnode)> 5*sizeof(struct mcs_spinlock)); >> + >> + if (!pv_enabled()) >> + return; >> + >> + pn->cpustate = PV_CPU_ACTIVE; >> + pn->mayhalt = false; >> + pn->mycpu = smp_processor_id(); >> + pn->head = PV_INVALID_HEAD; >> +} > >> @@ -333,6 +393,7 @@ queue: >> node += idx; >> node->locked = 0; >> node->next = NULL; >> + pv_init_node(node); >> >> /* >> * We touched a (possibly) cold cacheline in the per-cpu queue node; > > So even if !pv_enabled() the compiler will still have to emit the code > for that inline, which will generate additional register pressure, > icache pressure and lovely stuff like that. > > The patch I had used pv-ops for these things that would turn into NOPs > in the regular case and callee-saved function calls for the PV case. > > That still does not entirely eliminate cost, but does reduce it > significant. Please consider using that.The additional register pressure may just cause a few more register moves which should be negligible in the overall performance . The additional icache pressure, however, may have some impact on performance. I was trying to balance the performance of the pv and non-pv versions so that we won't penalize the pv code too much for a bit more performance in the non-pv code. Doing it your way will add a lot of function call and register saving/restoring to the pv code. Another alternative that I can think of is to generate 2 versions of the slowpath code - one pv and one non-pv out of the same source code. The non-pv code will call into the pv code once if pv is enabled. In this way, it won't increase the icache and register pressure of the non-pv code. However, this may make the source code a bit harder to read. Please let me know your thought on this alternate approach. -Longman
Peter Zijlstra
2014-Oct-24 22:04 UTC
[PATCH v12 09/11] pvqspinlock, x86: Add para-virtualization support
On Fri, Oct 24, 2014 at 04:53:27PM -0400, Waiman Long wrote:> The additional register pressure may just cause a few more register moves > which should be negligible in the overall performance . The additional > icache pressure, however, may have some impact on performance. I was trying > to balance the performance of the pv and non-pv versions so that we won't > penalize the pv code too much for a bit more performance in the non-pv code. > Doing it your way will add a lot of function call and register > saving/restoring to the pv code.If people care about performance they should not be using virt crap :-) I only really care about bare metal.
Mike Galbraith
2014-Oct-25 04:30 UTC
[PATCH v12 09/11] pvqspinlock, x86: Add para-virtualization support
On Sat, 2014-10-25 at 00:04 +0200, Peter Zijlstra wrote:> On Fri, Oct 24, 2014 at 04:53:27PM -0400, Waiman Long wrote: > > The additional register pressure may just cause a few more register moves > > which should be negligible in the overall performance . The additional > > icache pressure, however, may have some impact on performance. I was trying > > to balance the performance of the pv and non-pv versions so that we won't > > penalize the pv code too much for a bit more performance in the non-pv code. > > Doing it your way will add a lot of function call and register > > saving/restoring to the pv code. > > If people care about performance they should not be using virt crap :-)I tried some benching recently.. where did they hide the fastpaths? :) -Mike
Waiman Long
2014-Oct-27 17:15 UTC
[PATCH v12 09/11] pvqspinlock, x86: Add para-virtualization support
On 10/24/2014 06:04 PM, Peter Zijlstra wrote:> On Fri, Oct 24, 2014 at 04:53:27PM -0400, Waiman Long wrote: >> The additional register pressure may just cause a few more register moves >> which should be negligible in the overall performance . The additional >> icache pressure, however, may have some impact on performance. I was trying >> to balance the performance of the pv and non-pv versions so that we won't >> penalize the pv code too much for a bit more performance in the non-pv code. >> Doing it your way will add a lot of function call and register >> saving/restoring to the pv code. > If people care about performance they should not be using virt crap :-) > > I only really care about bare metal.Yes, I am aware of that. However, the whole point of doing PV spinlock is to improve performance in a virtual guest. Anyway, I had done some measurements. In my test system, the queue_spin_lock_slowpath() function has a text size of about 400 bytes without PV, but 1120 bytes with PV. I made some changes to create separate versions of PV and non-PV slowpath functions as shown by the diff below. The text size is now about 430 bytes for the non-PV version and 925 bytes for the PV version. The overall object size increases by a bit more than 200 bytes, but the icache footprint should be reduced no matter which version is used. -Longman ---------------------------------------- diff --git a/arch/x86/include/asm/pvqspinlock.h b/arch/x86/include/asm/pvqspinlo index d424252..241bf30 100644 --- a/arch/x86/include/asm/pvqspinlock.h +++ b/arch/x86/include/asm/pvqspinlock.h @@ -79,9 +79,6 @@ static inline void pv_init_node(struct mcs_spinlock *node) BUILD_BUG_ON(sizeof(struct pv_qnode) > 5*sizeof(struct mcs_spinlock)); - if (!pv_enabled()) - return; - pn->cpustate = PV_CPU_ACTIVE; pn->mayhalt = false; pn->mycpu = smp_processor_id(); @@ -132,9 +129,6 @@ static inline bool pv_link_and_wait_node(u32 old, struct mcs struct pv_qnode *ppn, *pn = (struct pv_qnode *)node; unsigned int count; - if (!pv_enabled()) - return false; - if (!(old & _Q_TAIL_MASK)) { node->locked = true; /* At queue head now */ goto ret; @@ -236,9 +230,6 @@ pv_wait_head(struct qspinlock *lock, struct mcs_spinlock *no { struct pv_qnode *pn = (struct pv_qnode *)node; - if (!pv_enabled()) - return smp_load_acquire(&lock->val.counter); - for (;;) { unsigned int count; s8 oldstate; @@ -328,8 +319,6 @@ static inline void pv_wait_check(struct qspinlock *lock, struct pv_qnode *pnxt = (struct pv_qnode *)next; struct pv_qnode *pcur = (struct pv_qnode *)node; - if (!pv_enabled()) - return; /* * Clear the locked and head values of lock holder */ diff --git a/kernel/locking/qspinlock.c b/kernel/locking/qspinlock.c index 1662dbd..05aea57 100644 --- a/kernel/locking/qspinlock.c +++ b/kernel/locking/qspinlock.c @@ -16,6 +16,7 @@ * Authors: Waiman Long <waiman.long at hp.com> * Peter Zijlstra <pzijlstr at redhat.com> */ +#ifndef _GEN_PV_LOCK_SLOWPATH #include <linux/smp.h> #include <linux/bug.h> #include <linux/cpumask.h> @@ -271,19 +272,37 @@ void queue_spin_unlock_slowpath(struct qspinlock *lock) } EXPORT_SYMBOL(queue_spin_unlock_slowpath); -#else +static void pv_queue_spin_lock_slowpath(struct qspinlock *lock, u32 val); + +#else /* CONFIG_PARAVIRT_SPINLOCKS */ + +static inline void pv_queue_spin_lock_slowpath(struct qspinlock *lock, u32 val) + { } -static inline void pv_init_node(struct mcs_spinlock *node) { } -static inline void pv_wait_check(struct qspinlock *lock, - struct mcs_spinlock *node, - struct mcs_spinlock *next) { } -static inline bool pv_link_and_wait_node(u32 old, struct mcs_spinlock *node) +#endif /* CONFIG_PARAVIRT_SPINLOCKS */ + +/* + * Dummy PV functions for bare-metal slowpath code + */ +static inline void nopv_init_node(struct mcs_spinlock *node) { } +static inline void nopv_wait_check(struct qspinlock *lock, + struct mcs_spinlock *node, + struct mcs_spinlock *next) { } +static inline bool nopv_link_and_wait_node(u32 old, struct mcs_spinlock *node) { return false; } -static inline int pv_wait_head(struct qspinlock *lock, +static inline int nopv_wait_head(struct qspinlock *lock, struct mcs_spinlock *node) { return smp_load_acquire(&lock->val.counter); } +static inline bool return_true(void) { return true; } +static inline bool return_false(void) { return false; } -#endif /* CONFIG_PARAVIRT_SPINLOCKS */ +#define pv_init_node nopv_init_node +#define pv_wait_check nopv_wait_check +#define pv_link_and_wait_node nopv_link_and_wait_node +#define pv_wait_head nopv_wait_head +#define in_pv_code return_false + +#endif /* _GEN_PV_LOCK_SLOWPATH */ /** * queue_spin_lock_slowpath - acquire the queue spinlock @@ -306,7 +325,11 @@ static inline int pv_wait_head(struct qspinlock *lock, * contended : (*,x,y) +--> (*,0,0) ---> (*,0,1) -' : * queue : ^--' : */ +#ifdef _GEN_PV_LOCK_SLOWPATH +static void pv_queue_spin_lock_slowpath(struct qspinlock *lock, u32 val) +#else void queue_spin_lock_slowpath(struct qspinlock *lock, u32 val) +#endif { struct mcs_spinlock *prev, *next, *node; u32 new, old, tail; @@ -314,7 +337,12 @@ void queue_spin_lock_slowpath(struct qspinlock *lock, u32 v BUILD_BUG_ON(CONFIG_NR_CPUS >= (1U << _Q_TAIL_CPU_BITS)); - if (pv_enabled()) + if (pv_enabled()) { + pv_queue_spin_lock_slowpath(lock, val); + return; + } + + if (in_pv_code()) goto queue; if (virt_queue_spin_lock(lock)) @@ -474,3 +502,23 @@ release: this_cpu_dec(mcs_nodes[0].count); } EXPORT_SYMBOL(queue_spin_lock_slowpath); + +#if !defined(_GEN_PV_LOCK_SLOWPATH) && defined(CONFIG_PARAVIRT_SPINLOCKS) +/* + * Generate the PV version of the queue_spin_lock_slowpath function + */ +#undef pv_init_node +#undef pv_wait_check +#undef pv_link_and_wait_node +#undef pv_wait_head +#undef EXPORT_SYMBOL +#undef in_pv_code + +#define _GEN_PV_LOCK_SLOWPATH +#define EXPORT_SYMBOL(x) +#define in_pv_code return_true +#define pv_enabled return_false + +#include "qspinlock.c" + +#endif
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