linux-kernel - [RFC] fpga: dfl: fme: Fix cpu hotplug code

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Date:   Mon, 28 Jun 2021 12:45:46 +0530
From:   Kajol Jain <kjain@...ux.ibm.com>
To:     will@...nel.org, hao.wu@...el.com, mark.rutland@....com
Cc:     trix@...hat.com, yilun.xu@...el.com, luwei.kang@...el.com,
        mdf@...nel.org, linux-fpga@...r.kernel.org,
        maddy@...ux.vnet.ibm.com, atrajeev@...ux.vnet.ibm.com,
        kjain@...ux.ibm.com, linux-kernel@...r.kernel.org,
        linuxppc-dev@...ts.ozlabs.org, rnsastry@...ux.ibm.com
Subject: [RFC] fpga: dfl: fme: Fix cpu hotplug code

Commit 724142f8c42a ("fpga: dfl: fme: add performance
reporting support") added performance reporting support
for FPGA management engine via perf.

It also added cpu hotplug feature but it didn't add
pmu migration call in cpu offline function.
This can create an issue incase the current designated
cpu being used to collect fme pmu data got offline,
as based on current code we are not migrating fme pmu to
new target cpu. Because of that perf will still try to
fetch data from that offline cpu and hence we will not
get counter data.

Patch fixed this issue by adding pmu_migrate_context call
in fme_perf_offline_cpu function.

Fixes: 724142f8c42a ("fpga: dfl: fme: add performance reporting support")
Signed-off-by: Kajol Jain <kjain@...ux.ibm.com>
---
 drivers/fpga/dfl-fme-perf.c | 4 ++++
 1 file changed, 4 insertions(+)

---
- This fix patch is not tested (as I don't have required environment).
  But issue mentioned in the commit msg can be re-created, by starting any
  fme_perf event and while its still running, offline current designated
  cpu pointed by cpumask file. Since current code didn't migrating pmu,
  perf gonna try getting counts from that offlined cpu and hence we will
  not get event data.
---
diff --git a/drivers/fpga/dfl-fme-perf.c b/drivers/fpga/dfl-fme-perf.c
index 4299145ef347..b9a54583e505 100644
--- a/drivers/fpga/dfl-fme-perf.c
+++ b/drivers/fpga/dfl-fme-perf.c
@@ -953,6 +953,10 @@ static int fme_perf_offline_cpu(unsigned int cpu, struct hlist_node *node)
 		return 0;
 
 	priv->cpu = target;
+
+	/* Migrate fme_perf pmu events to the new target cpu */
+	perf_pmu_migrate_context(&priv->pmu, cpu, target);
+
 	return 0;
 }
 
-- 
2.31.1