Re: [Patch v2 1/3] iommu: match the original algorithm

John Garry
2019-12-02T10:55:44Z

On 30/11/2019 05:58, Cong Wang wrote:
> On Fri, Nov 29, 2019 at 6:43 AM John Garry <john.garry@huawei.com> wrote:
>>
>> On 29/11/2019 00:48, Cong Wang wrote:
>>> The IOVA cache algorithm implemented in IOMMU code does not
>>> exactly match the original algorithm described in the paper.
>>>
>>
>> which paper?
>
> It's in drivers/iommu/iova.c, from line 769:
>
> 769 /*
> 770 * Magazine caches for IOVA ranges. For an introduction to magazines,
> 771 * see the USENIX 2001 paper "Magazines and Vmem: Extending the Slab
> 772 * Allocator to Many CPUs and Arbitrary Resources" by Bonwick and Adams.
> 773 * For simplicity, we use a static magazine size and don't implement the
> 774 * dynamic size tuning described in the paper.
> 775 */
>
>
>>
>>> Particularly, it doesn't need to free the loaded empty magazine
>>> when trying to put it back to global depot. To make it work, we
>>> have to pre-allocate magazines in the depot and only recycle them
>>> when all of them are full.
>>>
>>> Before this patch, rcache->depot[] contains either full or
>>> freed entries, after this patch, it contains either full or
>>> empty (but allocated) entries.
>>
>> I *quickly* tested this patch and got a small performance gain.
>
> Thanks for testing! It requires a different workload to see bigger gain,
> in our case, 24 memcache.parallel servers with 120 clients.
>

So in fact I was getting a ~10% throughput boost for my storage test.
Seems more than I would expect/hope for. I would need to test more.

>
>>
>>>
>>> Cc: Joerg Roedel <joro@8bytes.org>
>>> Signed-off-by: Cong Wang <xiyou.wangcong@gmail.com>
>>> ---
>>> drivers/iommu/iova.c | 45 +++++++++++++++++++++++++++-----------------
>>> 1 file changed, 28 insertions(+), 17 deletions(-)
>>>
>>> diff --git a/drivers/iommu/iova.c b/drivers/iommu/iova.c
>>> index 41c605b0058f..cb473ddce4cf 100644
>>> --- a/drivers/iommu/iova.c
>>> +++ b/drivers/iommu/iova.c
>>> @@ -862,12 +862,16 @@ static void init_iova_rcaches(struct iova_domain *iovad)
>>> struct iova_cpu_rcache *cpu_rcache;
>>> struct iova_rcache *rcache;
>>> unsigned int cpu;
>>> - int i;
>>> + int i, j;
>>>
>>> for (i = 0; i < IOVA_RANGE_CACHE_MAX_SIZE; ++i) {
>>> rcache = &iovad->rcaches[i];
>>> spin_lock_init(&rcache->lock);
>>> rcache->depot_size = 0;
>>> + for (j = 0; j < MAX_GLOBAL_MAGS; ++j) {
>>> + rcache->depot[j] = iova_magazine_alloc(GFP_KERNEL);
>>> + WARN_ON(!rcache->depot[j]);
>>> + }
>>> rcache->cpu_rcaches = __alloc_percpu(sizeof(*cpu_rcache), cache_line_size());
>>> if (WARN_ON(!rcache->cpu_rcaches))
>>> continue;
>>> @@ -900,24 +904,30 @@ static bool __iova_rcache_insert(struct iova_domain *iovad,
>>>
>>> if (!iova_magazine_full(cpu_rcache->loaded)) {
>>> can_insert = true;
>>> - } else if (!iova_magazine_full(cpu_rcache->prev)) {
>>> + } else if (iova_magazine_empty(cpu_rcache->prev)) {
>>
>> is this change strictly necessary?
>
> Yes, because it is what described in the paper. But it should be
> functionally same because cpu_rcache->prev is either full or empty.

That is was what I was getting at.

>
>
>
>>
>>> swap(cpu_rcache->prev, cpu_rcache->loaded);
>>> can_insert = true;
>>> } else {
>>> - struct iova_magazine *new_mag = iova_magazine_alloc(GFP_ATOMIC);

Apart from this change, did anyone ever consider kmem cache for the
magazines?

>>> + spin_lock(&rcache->lock);
>>> + if (rcache->depot_size < MAX_GLOBAL_MAGS) {
>>> + swap(rcache->depot[rcache->depot_size], cpu_rcache->prev);
>>> + swap(cpu_rcache->prev, cpu_rcache->loaded);
>>> + rcache->depot_size++;
>>> + can_insert = true;
>>> + } else {
>>> + mag_to_free = cpu_rcache->loaded;
>>> + }
>>> + spin_unlock(&rcache->lock);
>>> +
>>> + if (mag_to_free) {
>>> + struct iova_magazine *new_mag = iova_magazine_alloc(GFP_ATOMIC);
>>>
>>> - if (new_mag) {
>>> - spin_lock(&rcache->lock);
>>> - if (rcache->depot_size < MAX_GLOBAL_MAGS) {
>>> - rcache->depot[rcache->depot_size++] =
>>> - cpu_rcache->loaded;
>>> + if (new_mag) {
>>> + cpu_rcache->loaded = new_mag;
>>> + can_insert = true;
>>> } else {
>>> - mag_to_free = cpu_rcache->loaded;
>>> + mag_to_free = NULL;
>>> }
>>> - spin_unlock(&rcache->lock);
>>> -
>>> - cpu_rcache->loaded = new_mag;
>>> - can_insert = true;
>>> }
>>> }
>>>
>>> @@ -963,14 +973,15 @@ static unsigned long __iova_rcache_get(struct iova_rcache *rcache,
>>>
>>> if (!iova_magazine_empty(cpu_rcache->loaded)) {
>>> has_pfn = true;
>>> - } else if (!iova_magazine_empty(cpu_rcache->prev)) {
>>> + } else if (iova_magazine_full(cpu_rcache->prev)) {
>>> swap(cpu_rcache->prev, cpu_rcache->loaded);
>>> has_pfn = true;
>>> } else {
>>> spin_lock(&rcache->lock);
>>> if (rcache->depot_size > 0) {
>>> - iova_magazine_free(cpu_rcache->loaded);
>>
>> it is good to remove this from under the lock, apart from this change
>>
>>> - cpu_rcache->loaded = rcache->depot[--rcache->depot_size];
>>> + swap(rcache->depot[rcache->depot_size - 1], cpu_rcache->prev);
>>> + swap(cpu_rcache->prev, cpu_rcache->loaded);

I wonder if not using swap() at all is neater here.

>>> + rcache->depot_size--;
>>
>> I'm not sure how appropriate the name "depot_size" is any longer.
>
> I think it is still okay, because empty ones don't count. However if you
> have better names, I am open to your suggestion.

Yeah, probably.

thanks,
John