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Subject[PATCH] cxl: Use call_rcu to reduce latency when releasing the afu fd
Date
From: Ian Munsie <imunsie@au1.ibm.com>

The afu fd release path was identified as a significant bottleneck in
the overall performance of cxl. While an optimal AFU design would
minimise the need to close & reopen the AFU fd, it is not always
practical to avoid.

The bottleneck seems to be down to the call to synchronize_rcu(), which
will block until every other thread is guaranteed to be out of an RCU
critical section. Replace it with call_rcu() to free the context
structures later so we can return to the application sooner.

This reduces the time spent in the fd release path from 13356 usec to
13.3 usec - about a 100x speed up.

Reported-by: Fei K Chen <uchen@cn.ibm.com>
Signed-off-by: Ian Munsie <imunsie@au1.ibm.com>
---
drivers/misc/cxl/context.c | 15 ++++++++++-----
drivers/misc/cxl/cxl.h | 2 ++
2 files changed, 12 insertions(+), 5 deletions(-)

diff --git a/drivers/misc/cxl/context.c b/drivers/misc/cxl/context.c
index 22eb338..cea299e 100644
--- a/drivers/misc/cxl/context.c
+++ b/drivers/misc/cxl/context.c
@@ -243,12 +243,9 @@ void cxl_context_detach_all(struct cxl_afu *afu)
mutex_unlock(&afu->contexts_lock);
}

-void cxl_context_free(struct cxl_context *ctx)
+static void reclaim_ctx(struct rcu_head *rcu)
{
- mutex_lock(&ctx->afu->contexts_lock);
- idr_remove(&ctx->afu->contexts_idr, ctx->pe);
- mutex_unlock(&ctx->afu->contexts_lock);
- synchronize_rcu();
+ struct cxl_context *ctx = container_of(rcu, struct cxl_context, rcu);

free_page((u64)ctx->sstp);
ctx->sstp = NULL;
@@ -256,3 +253,11 @@ void cxl_context_free(struct cxl_context *ctx)
put_pid(ctx->pid);
kfree(ctx);
}
+
+void cxl_context_free(struct cxl_context *ctx)
+{
+ mutex_lock(&ctx->afu->contexts_lock);
+ idr_remove(&ctx->afu->contexts_idr, ctx->pe);
+ mutex_unlock(&ctx->afu->contexts_lock);
+ call_rcu(&ctx->rcu, reclaim_ctx);
+}
diff --git a/drivers/misc/cxl/cxl.h b/drivers/misc/cxl/cxl.h
index 47f655f..ebd2e0d 100644
--- a/drivers/misc/cxl/cxl.h
+++ b/drivers/misc/cxl/cxl.h
@@ -460,6 +460,8 @@ struct cxl_context {
bool pending_irq;
bool pending_fault;
bool pending_afu_err;
+
+ struct rcu_head rcu;
};

struct cxl {
--
2.1.4


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