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Re: [PATCH] xen/credit2: factor in previous active unit's credit in csched2_runtime()
- To: Koichiro Den <den@xxxxxxxxxxxxx>, xen-devel@xxxxxxxxxxxxxxxxxxxx
- From: Jürgen Groß <jgross@xxxxxxxx>
- Date: Fri, 25 Jul 2025 10:19:58 +0200
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- Cc: dfaggioli@xxxxxxxx, gwd@xxxxxxxxxxxxxx
- Delivery-date: Fri, 25 Jul 2025 08:20:13 +0000
- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
On 21.06.25 16:14, Koichiro Den wrote:
When a running unit is about to be scheduled out due to a competing unit
with the highest remaining credit, the residual credit of the previous
unit is currently ignored in csched2_runtime() because it hasn't yet
been reinserted into the runqueue.
As a result, two equally weighted, busy units can often each be granted
almost the maximum possible runtime (i.e. consuming CSCHED2_CREDIT_INIT
in one shot) when only those two are active. In broad strokes two units
switch back and forth every 10ms (CSCHED2_MAX_TIMER). In contrast, when
more than two busy units are competing, such coarse runtime allocations
are rarely seen, since at least one active unit remains in the runqueue.
To ensure consistent behavior, have csched2_runtime() take into account
the previous unit's latest credit when it still can/wants to run.
Signed-off-by: Koichiro Den <den@xxxxxxxxxxxxx>
Reviewed-by: Juergen Gross <jgross@xxxxxxxx>
Juergen
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