[xen master] xl: relax freemem()'s retry calculation

[patchbot@xxxxxxx]

commit e58370df76eacf1f7ca0340e9b96430c77b41a79
Author:     Jan Beulich <jbeulich@xxxxxxxx>
AuthorDate: Tue Jul 12 15:25:00 2022 +0200
Commit:     Jan Beulich <jbeulich@xxxxxxxx>
CommitDate: Tue Jul 12 15:25:00 2022 +0200

    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@xxxxxxxx>
    Reviewed-by: Anthony PERARD <anthony.perard@xxxxxxxxxx>
---
 tools/xl/xl_vmcontrol.c | 10 +++++++---
 1 file changed, 7 insertions(+), 3 deletions(-)

diff --git a/tools/xl/xl_vmcontrol.c b/tools/xl/xl_vmcontrol.c
index d081c6c290..cd338a5422 100644
--- 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;
 }
--
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