Re: [PATCH -next 2/3] md/raid10: convert resync_lock to use seqlock

From: Guoqing Jiang
Date: Fri Sep 02 2022 - 06:17:06 EST




On 9/2/22 6:02 PM, Yu Kuai wrote:
Hi,

在 2022/09/02 17:42, Guoqing Jiang 写道:
Hi,

On 8/29/22 9:15 PM, Yu Kuai wrote:
+static bool wait_barrier_nolock(struct r10conf *conf)
+{
+    unsigned int seq = raw_read_seqcount(&conf->resync_lock.seqcount);
+
+    if (seq & 1)
+        return false;
+
+    if (READ_ONCE(conf->barrier))
+        return false;
+
+    atomic_inc(&conf->nr_pending);
+    if (!read_seqcount_retry(&conf->resync_lock.seqcount, seq))

I think 'seq' is usually get from read_seqcount_begin.

read_seqcount_begin will loop untill "req & 1" failed, I'm afraid this
will cause high cpu usage in come cases.

What I try to do here is just try once, and fall back to hold lock and
wait if failed.

Thanks for the explanation.

I'd suggest to try with read_seqcount_begin/read_seqcount_retry pattern
because it is a common usage in kernel I think, then check whether the
performance drops or not.  Maybe it is related to lockdep issue, but I am
not sure.

Thanks,
Guoqing