Re: [Xen-devel] [PATCH] mem_event: use wait queue when ring is full

[Olaf Hering <olaf@xxxxxxxxx>]

On Thu, Dec 15, Andres Lagar-Cavilla wrote:

> > - How many requests should foreign vcpus place in the ring if the guest
> >   has more vcpus than available slots in the ring? Just a single one so
> >   that foreigners can also make some progress?
> The idea is that foreign vcpus can place as many events as they want as
> long as each guest vcpu that is not blocked on a men event has room to
> send one men event. When we reach that border condition, no more foreign
> men events.
> 
> The case for which there are way too many guest vcpus needs to be handled,
> either by capping the max number of vcpus for domains using a men event,
> or by growing the ring size.

Right now the ring is one page, so it can not hold more than 64 entries.
If that ever changes, the accounting can be adjusted.

> > - Should access and paging have the same rules for accounting?
> Absolutely.
> 
> And both should use wait queues in extreme cases in which a guest vcpu
> with a single action generates multiple memory events. Given that when we
> hit a border condition the guest vcpu will place one event and be flagged
> VPF_mem_event_paused (or whatever that flag is named), if a guest vcpu
> generates another event when flagged, that's our queue for putting the
> vcpu on a wait queue.

An extra flag is not needed.

Below is an incremental patch (on top of v6) which does some accounting.
Its just compile tested.

Olaf


diff -r 5d5d10e1568b xen/arch/x86/mm/mem_event.c
--- a/xen/arch/x86/mm/mem_event.c
+++ b/xen/arch/x86/mm/mem_event.c
@@ -114,6 +114,19 @@ static int mem_event_enable(
 
     med->pause_flag = pause_flag;
 
+    /*
+     * Configure ring accounting:
+     * Each guest vcpu should be able to place at least one request.
+     * If there are more vcpus than available slots in the ring, not all vcpus
+     * can place requests in the ring anyway.  A minimum (arbitrary) number of
+     * foreign requests will be allowed in this case.
+     */
+    if ( d->max_vcpus < RING_SIZE(&med->front_ring) )
+        med->max_foreign = RING_SIZE(&med->front_ring) - d->max_vcpus;
+    if ( med->max_foreign < 13 )
+        med->max_foreign = 13;
+    med->max_target = RING_SIZE(&med->front_ring) - med->max_foreign;
+
     init_waitqueue_head(&med->wq);
 
     /* Wake any VCPUs waiting for the ring to appear */
@@ -147,23 +160,28 @@ static int mem_event_disable(struct mem_
 
 static int _mem_event_put_request(struct domain *d,
                                   struct mem_event_domain *med,
-                                  mem_event_request_t *req)
+                                  mem_event_request_t *req,
+                                  int *done)
 {
     mem_event_front_ring_t *front_ring;
-    int free_req, claimed_req;
+    int free_req, claimed_req, ret;
     RING_IDX req_prod;
 
+    if ( *done )
+        return 1;
+
     mem_event_ring_lock(med);
 
-    free_req = RING_FREE_REQUESTS(&med->front_ring);
+    front_ring = &med->front_ring;
+
     /* Foreign requests must succeed because their vcpus can not sleep */
     claimed_req = med->foreign_producers;
+    free_req = RING_FREE_REQUESTS(front_ring);
     if ( !free_req || ( current->domain == d && free_req <= claimed_req ) ) {
         mem_event_ring_unlock(med);
         return 0;
     }
 
-    front_ring = &med->front_ring;
     req_prod = front_ring->req_prod_pvt;
 
     if ( current->domain != d )
@@ -176,9 +194,18 @@ static int _mem_event_put_request(struct
     memcpy(RING_GET_REQUEST(front_ring, req_prod), req, sizeof(*req));
     req_prod++;
 
+    ret = 1;
+    *done = 1;
+    free_req--;
+
     /* Update accounting */
     if ( current->domain == d )
+    {
         med->target_producers--;
+        /* Ring is full, no more requests from this vcpu, go to sleep */
+        if ( free_req < med->max_target )
+            ret = 0;
+    }
     else
         med->foreign_producers--;
 
@@ -190,19 +217,20 @@ static int _mem_event_put_request(struct
 
     notify_via_xen_event_channel(d, med->xen_port);
 
-    return 1;
+    return ret;
 }
 
 void mem_event_put_request(struct domain *d, struct mem_event_domain *med,
                            mem_event_request_t *req)
 {
+    int done = 0;
     /* Go to sleep if request came from guest */
     if (current->domain == d) {
-        wait_event(med->wq, _mem_event_put_request(d, med, req));
+        wait_event(med->wq, _mem_event_put_request(d, med, req, &done));
         return;
     }
     /* Ring was full anyway, unable to sleep in non-guest context */
-    if (!_mem_event_put_request(d, med, req))
+    if (!_mem_event_put_request(d, med, req, &done))
         printk("Failed to put memreq: d %u t %x f %x gfn %lx\n", d->domain_id,
                 req->type, req->flags, (unsigned long)req->gfn);
 }
@@ -341,7 +369,8 @@ int mem_event_claim_slot(struct domain *
         med->target_producers++;
         ring_full = 0;
     }
-    else if ( med->foreign_producers + med->target_producers + 1 < free_req )
+    else if ( med->foreign_producers + med->target_producers < free_req &&
+              med->foreign_producers < med->max_foreign )
     {
         med->foreign_producers++;
         ring_full = 0;
diff -r 5d5d10e1568b xen/include/xen/sched.h
--- a/xen/include/xen/sched.h
+++ b/xen/include/xen/sched.h
@@ -184,8 +184,11 @@ struct mem_event_domain
 {
     /* ring lock */
     spinlock_t ring_lock;
-    unsigned short foreign_producers;
-    unsigned short target_producers;
+    /* The ring has 64 entries */
+    unsigned char foreign_producers;
+    unsigned char max_foreign;
+    unsigned char target_producers;
+    unsigned char max_target;
     /* shared page */
     mem_event_shared_page_t *shared_page;
     /* shared ring page */

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