[xen master] memory: bail from page scrubbing when CPU is no longer online

[patchbot@xxxxxxx]

commit 3c9fd69416f8ffc611705fb24dfb383203ddc84f
Author:     Jan Beulich <jbeulich@xxxxxxxx>
AuthorDate: Fri Jan 29 11:34:37 2021 +0100
Commit:     Jan Beulich <jbeulich@xxxxxxxx>
CommitDate: Fri Jan 29 11:34:37 2021 +0100

    memory: bail from page scrubbing when CPU is no longer online
    
    Scrubbing can significantly delay the offlining (parking) of a CPU (e.g.
    because of booting into in smt=0 mode), to a degree that the "CPU <n>
    still not dead..." messages logged on x86 in 1s intervals can be seen
    multiple times. There are no softirqs involved in this process, so
    extend the existing preemption check in the scrubbing logic to also exit
    when the CPU is no longer observed online.
    
    Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
    Acked-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
---
 xen/common/page_alloc.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/xen/common/page_alloc.c b/xen/common/page_alloc.c
index fcb308d92f..02ac1fa613 100644
--- a/xen/common/page_alloc.c
+++ b/xen/common/page_alloc.c
@@ -1324,9 +1324,11 @@ bool scrub_free_pages(void)
                      * Scrub a few (8) pages before becoming eligible for
                      * preemption. But also count non-scrubbing loop iterations
                      * so that we don't get stuck here with an almost clean
-                     * heap.
+                     * heap. Consider the CPU no longer being seen as online as
+                     * a request to preempt immediately, to not unduly delay
+                     * its offlining.
                      */
-                    if ( cnt > 800 && softirq_pending(cpu) )
+                    if ( !cpu_online(cpu) || (cnt > 800 && 
softirq_pending(cpu)) )
                     {
                         preempt = true;
                         break;
--
generated by git-patchbot for /home/xen/git/xen.git#master