ceph: add trace points to the MDS client

This patch adds trace points to the Ceph filesystem MDS client:

- request submission (CEPH_MSG_CLIENT_REQUEST) and completion
(CEPH_MSG_CLIENT_REPLY)
- capabilities (CEPH_MSG_CLIENT_CAPS)

These are the central pieces that are useful for analyzing MDS
latency/performance problems from the client's perspective.

In the long run, all doutc() calls should be replaced with
tracepoints. This way, the Ceph filesystem can be traced at any time
(without spamming the kernel log). Additionally, trace points can be
used in BPF programs (which can even deference the pointer parameters
and extract more values).

Signed-off-by: Max Kellermann <[e-mail]>
Reviewed-by: Viacheslav Dubeyko <[e-mail]>
Signed-off-by: Ilya Dryomov <[e-mail]>

fs/ceph/caps.c

···
 #include "crypto.h"
 #include <linux/ceph/decode.h>
 #include <linux/ceph/messenger.h>
+#include <trace/events/ceph.h>
 
 /*
  * Capability management
···
 	doutc(cl, " caps mds%d op %s ino %llx.%llx inode %p seq %u iseq %u mseq %u\n",
 	      session->s_mds, ceph_cap_op_name(op), vino.ino, vino.snap, inode,
 	      seq, issue_seq, mseq);
+
+	trace_ceph_handle_caps(mdsc, session, op, &vino, ceph_inode(inode),
+			       seq, issue_seq, mseq);
 
 	mutex_lock(&session->s_mutex);
 

fs/ceph/mds_client.c

···
 #include <linux/ceph/pagelist.h>
 #include <linux/ceph/auth.h>
 #include <linux/ceph/debugfs.h>
+#include <trace/events/ceph.h>
 
 #define RECONNECT_MAX_SIZE (INT_MAX - PAGE_SIZE)
 
···
 {
 	req->r_end_latency = ktime_get();
 
+	trace_ceph_mdsc_complete_request(mdsc, req);
+
 	if (req->r_callback)
 		req->r_callback(mdsc, req);
 	complete_all(&req->r_completion);
···
 {
 	int err;
 
+	trace_ceph_mdsc_send_request(session, req);
+
 	err = __prepare_send_request(session, req, drop_cap_releases);
 	if (!err) {
 		ceph_msg_get(req->r_request);
···
 		}
 		if (mdsc->mdsmap->m_epoch == 0) {
 			doutc(cl, "no mdsmap, waiting for map\n");
+			trace_ceph_mdsc_suspend_request(mdsc, session, req,
+							ceph_mdsc_suspend_reason_no_mdsmap);
 			list_add(&req->r_wait, &mdsc->waiting_for_map);
 			return;
 		}
···
 			goto finish;
 		}
 		doutc(cl, "no mds or not active, waiting for map\n");
+		trace_ceph_mdsc_suspend_request(mdsc, session, req,
+						ceph_mdsc_suspend_reason_no_active_mds);
 		list_add(&req->r_wait, &mdsc->waiting_for_map);
 		return;
 	}
···
 		 * it to the mdsc queue.
 		 */
 		if (session->s_state == CEPH_MDS_SESSION_REJECTED) {
-			if (ceph_test_mount_opt(mdsc->fsc, CLEANRECOVER))
+			if (ceph_test_mount_opt(mdsc->fsc, CLEANRECOVER)) {
+				trace_ceph_mdsc_suspend_request(mdsc, session, req,
+								ceph_mdsc_suspend_reason_rejected);
 				list_add(&req->r_wait, &mdsc->waiting_for_map);
-			else
+			} else
 				err = -EACCES;
 			goto out_session;
 		}
···
 			if (random)
 				req->r_resend_mds = mds;
 		}
+		trace_ceph_mdsc_suspend_request(mdsc, session, req,
+						ceph_mdsc_suspend_reason_session);
 		list_add(&req->r_wait, &session->s_waiting);
 		goto out_session;
 	}
···
 		list_del_init(&req->r_wait);
 		doutc(cl, " wake request %p tid %llu\n", req,
 		      req->r_tid);
+		trace_ceph_mdsc_resume_request(mdsc, req);
 		__do_request(mdsc, req);
 	}
 }
···
 		    req->r_session->s_mds == mds) {
 			doutc(cl, " kicking tid %llu\n", req->r_tid);
 			list_del_init(&req->r_wait);
+			trace_ceph_mdsc_resume_request(mdsc, req);
 			__do_request(mdsc, req);
 		}
 	}
···
 	doutc(cl, "submit_request on %p for inode %p\n", req, dir);
 	mutex_lock(&mdsc->mutex);
 	__register_request(mdsc, req, dir);
+	trace_ceph_mdsc_submit_request(mdsc, req);
 	__do_request(mdsc, req);
 	err = req->r_err;
 	mutex_unlock(&mdsc->mutex);

fs/ceph/super.c

···
 
 #include <uapi/linux/magic.h>
 
+#define CREATE_TRACE_POINTS
+#include <trace/events/ceph.h>
+
 static DEFINE_SPINLOCK(ceph_fsc_lock);
 static LIST_HEAD(ceph_fsc_list);
 

include/trace/events/ceph.h

···
+/* SPDX-License-Identifier: GPL-2.0-or-later */
+/* Ceph filesystem support module tracepoints
+ *
+ * Copyright (C) 2025 IONOS SE. All Rights Reserved.
+ * Written by Max Kellermann ([e-mail])
+ */
+#undef TRACE_SYSTEM
+#define TRACE_SYSTEM ceph
+
+#if !defined(_TRACE_CEPH_H) || defined(TRACE_HEADER_MULTI_READ)
+#define _TRACE_CEPH_H
+
+#include <linux/tracepoint.h>
+
+#define ceph_mdsc_suspend_reasons						\
+	EM(ceph_mdsc_suspend_reason_no_mdsmap,		"no-mdsmap")		\
+	EM(ceph_mdsc_suspend_reason_no_active_mds,	"no-active-mds")	\
+	EM(ceph_mdsc_suspend_reason_rejected,		"rejected")		\
+	E_(ceph_mdsc_suspend_reason_session,		"session")
+
+#ifndef __NETFS_DECLARE_TRACE_ENUMS_ONCE_ONLY
+#define __NETFS_DECLARE_TRACE_ENUMS_ONCE_ONLY
+
+#undef EM
+#undef E_
+#define EM(a, b) a,
+#define E_(a, b) a
+
+enum ceph_mdsc_suspend_reason { ceph_mdsc_suspend_reasons } __mode(byte);
+
+#endif
+
+/*
+ * Export enum symbols via userspace.
+ */
+#undef EM
+#undef E_
+#define EM(a, b) TRACE_DEFINE_ENUM(a);
+#define E_(a, b) TRACE_DEFINE_ENUM(a);
+
+ceph_mdsc_suspend_reasons;
+
+/*
+ * Now redefine the EM() and E_() macros to map the enums to the strings that
+ * will be printed in the output.
+ */
+#undef EM
+#undef E_
+#define EM(a, b)	{ a, b },
+#define E_(a, b)	{ a, b }
+
+TRACE_EVENT(ceph_mdsc_submit_request,
+	TP_PROTO(struct ceph_mds_client *mdsc,
+		 struct ceph_mds_request *req),
+
+	TP_ARGS(mdsc, req),
+
+	TP_STRUCT__entry(
+		__field(u64,	tid)
+		__field(int,	op)
+		__field(u64,	ino)
+		__field(u64,	snap)
+	),
+
+	TP_fast_assign(
+		struct inode *inode;
+
+		__entry->tid = req->r_tid;
+		__entry->op = req->r_op;
+
+		inode = req->r_inode;
+		if (inode == NULL && req->r_dentry)
+			inode = d_inode(req->r_dentry);
+
+		if (inode) {
+			__entry->ino = ceph_ino(inode);
+			__entry->snap = ceph_snap(inode);
+		} else {
+			__entry->ino = __entry->snap = 0;
+		}
+	),
+
+	TP_printk("R=%llu op=%s ino=%llx,%llx",
+		  __entry->tid,
+		  ceph_mds_op_name(__entry->op),
+		  __entry->ino, __entry->snap)
+);
+
+TRACE_EVENT(ceph_mdsc_suspend_request,
+	TP_PROTO(struct ceph_mds_client *mdsc,
+		     struct ceph_mds_session *session,
+		     struct ceph_mds_request *req,
+		     enum ceph_mdsc_suspend_reason reason),
+
+	TP_ARGS(mdsc, session, req, reason),
+
+	TP_STRUCT__entry(
+		__field(u64,				tid)
+		__field(int,				op)
+		__field(int,				mds)
+		__field(enum ceph_mdsc_suspend_reason,	reason)
+	),
+
+	TP_fast_assign(
+		__entry->tid = req->r_tid;
+		__entry->op = req->r_op;
+		__entry->mds = session ? session->s_mds : -1;
+		__entry->reason = reason;
+	),
+
+	TP_printk("R=%llu op=%s reason=%s",
+		  __entry->tid,
+		  ceph_mds_op_name(__entry->op),
+		  __print_symbolic(__entry->reason, ceph_mdsc_suspend_reasons))
+);
+
+TRACE_EVENT(ceph_mdsc_resume_request,
+	TP_PROTO(struct ceph_mds_client *mdsc,
+		 struct ceph_mds_request *req),
+
+	TP_ARGS(mdsc, req),
+
+	TP_STRUCT__entry(
+		__field(u64,				tid)
+		__field(int,				op)
+	),
+
+	TP_fast_assign(
+		__entry->tid = req->r_tid;
+		__entry->op = req->r_op;
+	),
+
+	TP_printk("R=%llu op=%s",
+		  __entry->tid,
+		  ceph_mds_op_name(__entry->op))
+);
+
+TRACE_EVENT(ceph_mdsc_send_request,
+	TP_PROTO(struct ceph_mds_session *session,
+		 struct ceph_mds_request *req),
+
+	TP_ARGS(session, req),
+
+	TP_STRUCT__entry(
+		__field(u64,		tid)
+		__field(int,		op)
+		__field(int,		mds)
+	),
+
+	TP_fast_assign(
+		__entry->tid = req->r_tid;
+		__entry->op = req->r_op;
+		__entry->mds = session->s_mds;
+	),
+
+	TP_printk("R=%llu op=%s mds=%d",
+		  __entry->tid,
+		  ceph_mds_op_name(__entry->op),
+		  __entry->mds)
+);
+
+TRACE_EVENT(ceph_mdsc_complete_request,
+	TP_PROTO(struct ceph_mds_client *mdsc,
+		     struct ceph_mds_request *req),
+
+	TP_ARGS(mdsc, req),
+
+	TP_STRUCT__entry(
+		__field(u64,			tid)
+		__field(int,			op)
+		__field(int,			err)
+		__field(unsigned long,		latency_ns)
+	),
+
+	TP_fast_assign(
+		__entry->tid = req->r_tid;
+		__entry->op = req->r_op;
+		__entry->err = req->r_err;
+		__entry->latency_ns = req->r_end_latency - req->r_start_latency;
+	),
+
+	TP_printk("R=%llu op=%s err=%d latency_ns=%lu",
+		  __entry->tid,
+		  ceph_mds_op_name(__entry->op),
+		  __entry->err,
+		  __entry->latency_ns)
+);
+
+TRACE_EVENT(ceph_handle_caps,
+	TP_PROTO(struct ceph_mds_client *mdsc,
+		 struct ceph_mds_session *session,
+		 int op,
+		 const struct ceph_vino *vino,
+		 struct ceph_inode_info *inode,
+		 u32 seq, u32 mseq, u32 issue_seq),
+
+	TP_ARGS(mdsc, session, op, vino, inode, seq, mseq, issue_seq),
+
+	TP_STRUCT__entry(
+		__field(int,	mds)
+		__field(int,	op)
+		__field(u64,	ino)
+		__field(u64,	snap)
+		__field(u32,	seq)
+		__field(u32,	mseq)
+		__field(u32,	issue_seq)
+	),
+
+	TP_fast_assign(
+		__entry->mds = session->s_mds;
+		__entry->op = op;
+		__entry->ino = vino->ino;
+		__entry->snap = vino->snap;
+		__entry->seq = seq;
+		__entry->mseq = mseq;
+		__entry->issue_seq = issue_seq;
+	),
+
+	TP_printk("mds=%d op=%s vino=%llx.%llx seq=%u iseq=%u mseq=%u",
+		  __entry->mds,
+		  ceph_cap_op_name(__entry->op),
+		  __entry->ino,
+		  __entry->snap,
+		  __entry->seq,
+		  __entry->issue_seq,
+		  __entry->mseq)
+);
+
+#undef EM
+#undef E_
+#endif /* _TRACE_CEPH_H */
+
+/* This part must be outside protection */
+#include <trace/define_trace.h>