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Re: [PATCH] xen/balloon: use a kernel thread instead a workqueue

On 08.09.21 16:47, Boris Ostrovsky wrote:

On 8/27/21 8:32 AM, Juergen Gross wrote:
+static bool balloon_thread_cond(enum bp_state state, long credit)
+       if (state != BP_EAGAIN)
+               credit = 0;
+       return current_credit() != credit || kthread_should_stop();
+ * As this is a kthread it is guaranteed to run as a single instance only.
   * We may of course race updates of the target counts (which are protected
   * by the balloon lock), or with changes to the Xen hard limit, but we will
   * recover from these in time.
-static void balloon_process(struct work_struct *work)
+static int balloon_thread(void *unused)
        enum bp_state state = BP_DONE;
        long credit;
+       unsigned long timeout;
+       set_freezable();
+       for (;;) {
+               if (state == BP_EAGAIN)
+                       timeout = balloon_stats.schedule_delay * HZ;
+               else
+                       timeout = 3600 * HZ;
+               credit = current_credit();
+ wait_event_interruptible_timeout(balloon_thread_wq,
+                                balloon_thread_cond(state, credit), timeout);

Given that wait_event_interruptible_timeout() is a bunch of nested macros do we 
need to worry here about overly aggressive compiler optimizing out 'credit = 

I don't think so. current_credit() is reading from balloon_stats, which
is a global variable. So the compiler shouldn't assume the contents
won't change.

But I can add a barrier() after 'credit = current_credit()' in case
you'd feel uneasy without it.


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