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cxl: Use call_rcu to reduce latency when releasing the afu fd

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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>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
This commit is contained in:
Ian Munsie 2015-05-08 22:55:18 +10:00 committed by Michael Ellerman
parent e36f6fe1f7
commit 8ac75b96be
2 changed files with 12 additions and 5 deletions
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15
drivers/misc/cxl/context.c
View file

@ -232,12 +232,9 @@ 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;
@ -245,3 +242,11 @@ 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);
}

2
drivers/misc/cxl/cxl.h
View file

@ -457,6 +457,8 @@ struct cxl_context {
bool pending_irq;
bool pending_fault;
bool pending_afu_err;
struct rcu_head rcu;
};
struct cxl {