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Date: Wed, 11 Oct 2023 19:24:42 +0800
From: Like Xu <like.xu.linux@...il.com>
To: Alex Williamson <alex.williamson@...hat.com>
Cc: Paolo Bonzini <pbonzini@...hat.com>, linux-kernel@...r.kernel.org,
kvm@...r.kernel.org, Sean Christopherson <seanjc@...gle.com>
Subject: Re: [PATCH v2 2/2] KVM: irqbypass: Convert producers/consumers single
linked list to XArray
Hi all, do we have any negative comments on this issue ?
This register path could be a performance bottleneck in serverless scenarios.
Your comments are much appreciated, thanks!
On 26/9/2023 5:18 pm, Like Xu wrote:
> On 25/9/2023 11:24 pm, Like Xu wrote:
>>>> @@ -97,24 +98,23 @@ int irq_bypass_register_producer(struct
>>>> irq_bypass_producer *producer)
>>>> mutex_lock(&lock);
>>>> - list_for_each_entry(tmp, &producers, node) {
>>>> - if (tmp->token == producer->token || tmp == producer) {
>>>> - ret = -EBUSY;
>>>> + tmp = xa_load(&producers, token);
>>>> + if (tmp || tmp == producer) {
>>>> + ret = -EBUSY;
>>>> + goto out_err;
>>>> + }
>>>> +
>>>> + ret = xa_err(xa_store(&producers, token, producer, GFP_KERNEL));
>>>> + if (ret)
>>>> + goto out_err;
>>>> +
>>>> + consumer = xa_load(&consumers, token);
>>>> + if (consumer) {
>>>> + ret = __connect(producer, consumer);
>>>> + if (ret)
>>>> goto out_err;
>>>
>>> This doesn't match previous behavior, the producer is registered to the
>>> xarray regardless of the result of the connect operation and the caller
>>> cannot distinguish between failures. The module reference is released
>>> regardless of xarray item. Nak.
>>
>> Hi Alex,
>>
>> Thanks for your comments and indeed, the additional error throwing logic
>> breaks the caller's expectations as you said.
>>
>> What if we use LIST as a fallback option for XARRAY? Specifically, when
>> xa_err(xa_store()) is true, then fallback to use LIST to check for
>> producers/consumers, and in most cases it still takes the XARRAY path:
>>
>> static DEFINE_XARRAY(xproducers);
>> ...
>> if (xa_err(xa_store(&xproducers, (unsigned long)producer->token,
>> producer, GFP_KERNEL)))
>> list_add(&producer->node, &producers);
>> ...
>>
>> There will also be a LIST option on the lookup path.
>>
>> The rough code already works, could we move in this direction (combining
>> XARRAY with LIST to hidden the memory allocation error from xa_store) ?
>
> For better discussion and further improvement, here's the draft code combining
> xarray and list, using both xarray and list to store producers and consumers,
> but with xarray preferred for queries:
>
> diff --git a/virt/lib/irqbypass.c b/virt/lib/irqbypass.c
> index e0aabbbf27ec..7cc30d699ece 100644
> --- a/virt/lib/irqbypass.c
> +++ b/virt/lib/irqbypass.c
> @@ -18,12 +18,15 @@
> #include <linux/list.h>
> #include <linux/module.h>
> #include <linux/mutex.h>
> +#include <linux/xarray.h>
>
> MODULE_LICENSE("GPL v2");
> MODULE_DESCRIPTION("IRQ bypass manager utility module");
>
> static LIST_HEAD(producers);
> static LIST_HEAD(consumers);
> +static DEFINE_XARRAY(xproducers);
> +static DEFINE_XARRAY(xconsumers);
> static DEFINE_MUTEX(lock);
>
> /* @lock must be held when calling connect */
> @@ -74,6 +77,117 @@ static void __disconnect(struct irq_bypass_producer *prod,
> prod->start(prod);
> }
>
> +#define CHECK_TOKEN BIT_ULL(0)
> +#define CHECK_POINTER BIT_ULL(1)
> +
> +static inline bool
> +producer_already_exist(struct irq_bypass_producer *producer, u64 flags)
> +{
> + struct irq_bypass_producer *tmp;
> +
> + if (((flags & CHECK_POINTER) && xa_load(&xproducers,
> + (unsigned long)producer)) ||
> + ((flags & CHECK_TOKEN) && xa_load(&xproducers,
> + (unsigned long)producer->token)))
> + return true;
> +
> + list_for_each_entry(tmp, &producers, node) {
> + if (((flags & CHECK_POINTER) && tmp == producer) ||
> + ((flags & CHECK_TOKEN) && tmp->token == producer->token))
> + return true;
> + }
> +
> + return false;
> +}
> +
> +static inline bool
> +consumer_already_exist(struct irq_bypass_consumer *consumer, u64 flags)
> +{
> + struct irq_bypass_consumer *tmp;
> +
> + if (((flags & CHECK_POINTER) && xa_load(&xconsumers,
> + (unsigned long)consumer)) ||
> + ((flags & CHECK_TOKEN) && xa_load(&xconsumers,
> + (unsigned long)consumer->token)))
> + return true;
> +
> + list_for_each_entry(tmp, &consumers, node) {
> + if (((flags & CHECK_POINTER) && tmp == consumer) ||
> + ((flags & CHECK_TOKEN) && tmp->token == consumer->token))
> + return true;
> + }
> +
> + return false;
> +}
> +
> +static inline struct irq_bypass_producer *get_producer_by_token(void *token)
> +{
> + struct irq_bypass_producer *tmp;
> +
> + tmp = xa_load(&xproducers, (unsigned long)token);
> + if (tmp)
> + return tmp;
> +
> + list_for_each_entry(tmp, &producers, node) {
> + if (tmp->token == token)
> + return tmp;
> + }
> +
> + return NULL;
> +}
> +
> +static inline struct irq_bypass_consumer *get_consumer_by_token(void *token)
> +{
> + struct irq_bypass_consumer *tmp;
> +
> + tmp = xa_load(&xconsumers, (unsigned long)token);
> + if (tmp)
> + return tmp;
> +
> + list_for_each_entry(tmp, &consumers, node) {
> + if (tmp->token == token)
> + return tmp;
> + }
> +
> + return NULL;
> +}
> +
> +static inline void add_irq_bypass_producer(struct irq_bypass_producer *producer)
> +{
> + xa_store(&xproducers, (unsigned long)producer->token,
> + producer, GFP_KERNEL);
> + xa_store(&xproducers, (unsigned long)producer,
> + producer, GFP_KERNEL);
> +
> + list_add(&producer->node, &producers);
> +}
> +
> +static inline void del_irq_bypass_producer(struct irq_bypass_producer *producer)
> +{
> + xa_erase(&xproducers, (unsigned long)producer->token);
> + xa_erase(&xproducers, (unsigned long)producer);
> +
> + list_del(&producer->node);
> +}
> +
> +static inline void add_irq_bypass_consumer(struct irq_bypass_consumer *consumer)
> +{
> + xa_store(&xconsumers, (unsigned long)consumer->token,
> + consumer, GFP_KERNEL);
> + xa_store(&xconsumers, (unsigned long)consumer,
> + consumer, GFP_KERNEL);
> +
> + list_add(&consumer->node, &consumers);
> +}
> +
> +static inline void del_irq_bypass_consumer(struct irq_bypass_consumer *consumer)
> +{
> + xa_erase(&xconsumers, (unsigned long)consumer->token);
> + xa_erase(&xconsumers, (unsigned long)consumer);
> +
> + list_del(&consumer->node);
> +}
> +
> /**
> * irq_bypass_register_producer - register IRQ bypass producer
> * @producer: pointer to producer structure
> @@ -83,7 +197,6 @@ static void __disconnect(struct irq_bypass_producer *prod,
> */
> int irq_bypass_register_producer(struct irq_bypass_producer *producer)
> {
> - struct irq_bypass_producer *tmp;
> struct irq_bypass_consumer *consumer;
> int ret;
>
> @@ -97,23 +210,19 @@ int irq_bypass_register_producer(struct irq_bypass_producer
> *producer)
>
> mutex_lock(&lock);
>
> - list_for_each_entry(tmp, &producers, node) {
> - if (tmp->token == producer->token || tmp == producer) {
> - ret = -EBUSY;
> + if (producer_already_exist(producer, CHECK_TOKEN | CHECK_POINTER)) {
> + ret = -EBUSY;
> + goto out_err;
> + }
> +
> + consumer = get_consumer_by_token(producer->token);
> + if (consumer) {
> + ret = __connect(producer, consumer);
> + if (ret)
> goto out_err;
> - }
> }
>
> - list_for_each_entry(consumer, &consumers, node) {
> - if (consumer->token == producer->token) {
> - ret = __connect(producer, consumer);
> - if (ret)
> - goto out_err;
> - break;
> - }
> - }
> -
> - list_add(&producer->node, &producers);
> + add_irq_bypass_producer(producer);
>
> mutex_unlock(&lock);
>
> @@ -134,7 +243,6 @@ EXPORT_SYMBOL_GPL(irq_bypass_register_producer);
> */
> void irq_bypass_unregister_producer(struct irq_bypass_producer *producer)
> {
> - struct irq_bypass_producer *tmp;
> struct irq_bypass_consumer *consumer;
>
> if (!producer->token)
> @@ -147,20 +255,13 @@ void irq_bypass_unregister_producer(struct
> irq_bypass_producer *producer)
>
> mutex_lock(&lock);
>
> - list_for_each_entry(tmp, &producers, node) {
> - if (tmp != producer)
> - continue;
> + if (producer_already_exist(producer, CHECK_POINTER)) {
> + consumer = get_consumer_by_token(producer->token);
> + if (consumer)
> + __disconnect(producer, consumer);
>
> - list_for_each_entry(consumer, &consumers, node) {
> - if (consumer->token == producer->token) {
> - __disconnect(producer, consumer);
> - break;
> - }
> - }
> -
> - list_del(&producer->node);
> + del_irq_bypass_producer(producer);
> module_put(THIS_MODULE);
> - break;
> }
>
> mutex_unlock(&lock);
> @@ -178,7 +279,6 @@ EXPORT_SYMBOL_GPL(irq_bypass_unregister_producer);
> */
> int irq_bypass_register_consumer(struct irq_bypass_consumer *consumer)
> {
> - struct irq_bypass_consumer *tmp;
> struct irq_bypass_producer *producer;
> int ret;
>
> @@ -193,23 +293,19 @@ int irq_bypass_register_consumer(struct
> irq_bypass_consumer *consumer)
>
> mutex_lock(&lock);
>
> - list_for_each_entry(tmp, &consumers, node) {
> - if (tmp->token == consumer->token || tmp == consumer) {
> - ret = -EBUSY;
> + if (consumer_already_exist(consumer, CHECK_TOKEN | CHECK_POINTER)) {
> + ret = -EBUSY;
> + goto out_err;
> + }
> +
> + producer = get_producer_by_token(consumer->token);
> + if (producer) {
> + ret = __connect(producer, consumer);
> + if (ret)
> goto out_err;
> - }
> }
>
> - list_for_each_entry(producer, &producers, node) {
> - if (producer->token == consumer->token) {
> - ret = __connect(producer, consumer);
> - if (ret)
> - goto out_err;
> - break;
> - }
> - }
> -
> - list_add(&consumer->node, &consumers);
> + add_irq_bypass_consumer(consumer);
>
> mutex_unlock(&lock);
>
> @@ -230,7 +326,6 @@ EXPORT_SYMBOL_GPL(irq_bypass_register_consumer);
> */
> void irq_bypass_unregister_consumer(struct irq_bypass_consumer *consumer)
> {
> - struct irq_bypass_consumer *tmp;
> struct irq_bypass_producer *producer;
>
> if (!consumer->token)
> @@ -243,20 +338,13 @@ void irq_bypass_unregister_consumer(struct
> irq_bypass_consumer *consumer)
>
> mutex_lock(&lock);
>
> - list_for_each_entry(tmp, &consumers, node) {
> - if (tmp != consumer)
> - continue;
> + if (consumer_already_exist(consumer, CHECK_POINTER)) {
> + producer = get_producer_by_token(consumer->token);
> + if (producer)
> + __disconnect(producer, consumer);
>
> - list_for_each_entry(producer, &producers, node) {
> - if (producer->token == consumer->token) {
> - __disconnect(producer, consumer);
> - break;
> - }
> - }
> -
> - list_del(&consumer->node);
> + del_irq_bypass_consumer(consumer);
> module_put(THIS_MODULE);
> - break;
> }
>
> mutex_unlock(&lock);
> @@ -264,3 +352,10 @@ void irq_bypass_unregister_consumer(struct
> irq_bypass_consumer *consumer)
> module_put(THIS_MODULE);
> }
> EXPORT_SYMBOL_GPL(irq_bypass_unregister_consumer);
> +
> +static void __exit irqbypass_exit(void)
> +{
> + xa_destroy(&xproducers);
> + xa_destroy(&xconsumers);
> +}
> +module_exit(irqbypass_exit);
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