xl: relax freemem()'s retry calculation

While in principle possible also under other conditions as long as other
parallel operations potentially consuming memory aren't "locked out", in
particular with IOMMU large page mappings used in Dom0 (for PV when in
strict mode; for PVH when not sharing page tables with HAP) ballooning
out of individual pages can actually lead to less free memory available
afterwards. This is because to split a large page, one or more page
table pages are necessary (one per level that is split).

When rebooting a guest I've observed freemem() to fail: A single page
was required to be ballooned out (presumably because of heap
fragmentation in the hypervisor). This ballooning out of a single page
of course went fast, but freemem() then found that it would require to
balloon out another page. This repeating just another time leads to the
function to signal failure to the caller - without having come anywhere
near the designated 30s that the whole process is allowed to not make
any progress at all.

Convert from a simple retry count to actually calculating elapsed time,
subtracting from an initial credit of 30s. Don't go as far as limiting
the "wait_secs" value passed to libxl_wait_for_memory_target(), though.
While this leads to the overall process now possibly taking longer (if
the previous iteration ended very close to the intended 30s), this
compensates to some degree for the value passed really meaning "allowed
to run for this long without making progress".

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Anthony PERARD <anthony.perard@citrix.com>

diff --git a/tools/xl/xl_vmcontrol.c b/tools/xl/xl_vmcontrol.c
index d081c6c29092aa2a377fc59e1bbb0c41b6307802..cd338a5422ea15f18bed70cfc5badc06a789d7ad 100644 (file)
--- a/tools/xl/xl_vmcontrol.c
+++ b/tools/xl/xl_vmcontrol.c
@@ -321,7 +321,8 @@ static int domain_wait_event(uint32_t domid, libxl_event **event_r)
  */
 static bool freemem(uint32_t domid, libxl_domain_config *d_config)
 {
-    int rc, retries = 3;
+    int rc;
+    double credit = 30;
     uint64_t need_memkb, free_memkb;
 
     if (!autoballoon)
@@ -332,6 +333,8 @@ static bool freemem(uint32_t domid, libxl_domain_config *d_config)
         return false;
 
     do {
+        time_t start;
+
         rc = libxl_get_free_memory(ctx, &free_memkb);
         if (rc < 0)
             return false;
@@ -345,12 +348,13 @@ static bool freemem(uint32_t domid, libxl_domain_config *d_config)
 
         /* wait until dom0 reaches its target, as long as we are making
          * progress */
+        start = time(NULL);
         rc = libxl_wait_for_memory_target(ctx, 0, 10);
         if (rc < 0)
             return false;
 
-        retries--;
-    } while (retries > 0);
+        credit -= difftime(time(NULL), start);
+    } while (credit > 0);
 
     return false;
 }