From patchwork Fri Dec 27 15:13:52 2024 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit X-Patchwork-Submitter: Steven Rostedt X-Patchwork-Id: 13921954 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-1.web.codeaurora.org [10.30.226.201]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 555B01F76D0; Fri, 27 Dec 2024 15:13:02 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=10.30.226.201 ARC-Seal: i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1735312382; cv=none; b=fZdjmD0AGlDH1drzxOkzuoW+/Y4rUBWW1iU2KdVt6zV2zLn1SqTAiecmmYeiGV/mrI0P/mh6TMp2jc9CidxfO3d3xyGQxDZ57P24hqeG+jkWYFgGmibuO1Yna5W+1LTHV3karH45N+ENTaQGx2YuzKmC39cZtk9PgzPI8WLGnjw= ARC-Message-Signature: i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1735312382; c=relaxed/simple; bh=/U2u8EsBjB6KIchgLUIQ+C3gcHKRKnTVN5kskepLzL4=; h=Message-ID:Date:From:To:Cc:Subject:References:MIME-Version: Content-Type; b=Z839+aKg/P3QB49bXc8otQ/Zwx9BKfOeyC9mpwRs1AAWBsjQd/mDAJ6RU/WpMK+yDvCbCJ1nCCICh5wGU3T9ajr24UeGFyMpYIYFhEPz7Y+5uhd6fMQvW/+DwlCy465Kh1aIbMvJxfwwh3V2OcKAh8l0pL5Zbv792mQHooUvJjM= ARC-Authentication-Results: i=1; smtp.subspace.kernel.org; arc=none smtp.client-ip=10.30.226.201 Received: by smtp.kernel.org (Postfix) with ESMTPSA id 1EA5AC4CEE2; Fri, 27 Dec 2024 15:13:02 +0000 (UTC) Received: from rostedt by gandalf with local (Exim 4.98) (envelope-from ) id 1tRC2N-0000000GxaS-2aLy; Fri, 27 Dec 2024 10:14:03 -0500 Message-ID: <20241227151403.466542720@goodmis.org> User-Agent: quilt/0.68 Date: Fri, 27 Dec 2024 10:13:52 -0500 From: Steven Rostedt To: linux-kernel@vger.kernel.org Cc: Masami Hiramatsu , Mark Rutland , Mathieu Desnoyers , Andrew Morton , Alexei Starovoitov , Florent Revest , Martin KaFai Lau , bpf , Alexei Starovoitov , Jiri Olsa , Alan Maguire Subject: [for-next][PATCH 17/18] Documentation: probes: Update fprobe on function-graph tracer References: <20241227151335.898746489@goodmis.org> Precedence: bulk X-Mailing-List: bpf@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 From: "Masami Hiramatsu (Google)" Update fprobe documentation for the new fprobe on function-graph tracer. This includes some bahvior changes and pt_regs to ftrace_regs interface change. Cc: Alexei Starovoitov Cc: Florent Revest Cc: Martin KaFai Lau Cc: bpf Cc: Alexei Starovoitov Cc: Jiri Olsa Cc: Alan Maguire Cc: Mark Rutland Link: https://lore.kernel.org/173519010442.391279.10732749889346824783.stgit@devnote2 Signed-off-by: Masami Hiramatsu (Google) Signed-off-by: Steven Rostedt (Google) --- Documentation/trace/fprobe.rst | 42 ++++++++++++++++++++++------------ 1 file changed, 27 insertions(+), 15 deletions(-) diff --git a/Documentation/trace/fprobe.rst b/Documentation/trace/fprobe.rst index 196f52386aaa..71cd40472d36 100644 --- a/Documentation/trace/fprobe.rst +++ b/Documentation/trace/fprobe.rst @@ -9,9 +9,10 @@ Fprobe - Function entry/exit probe Introduction ============ -Fprobe is a function entry/exit probe mechanism based on ftrace. -Instead of using ftrace full feature, if you only want to attach callbacks -on function entry and exit, similar to the kprobes and kretprobes, you can +Fprobe is a function entry/exit probe based on the function-graph tracing +feature in ftrace. +Instead of tracing all functions, if you want to attach callbacks on specific +function entry and exit, similar to the kprobes and kretprobes, you can use fprobe. Compared with kprobes and kretprobes, fprobe gives faster instrumentation for multiple functions with single handler. This document describes how to use fprobe. @@ -91,12 +92,14 @@ The prototype of the entry/exit callback function are as follows: .. code-block:: c - int entry_callback(struct fprobe *fp, unsigned long entry_ip, unsigned long ret_ip, struct pt_regs *regs, void *entry_data); + int entry_callback(struct fprobe *fp, unsigned long entry_ip, unsigned long ret_ip, struct ftrace_regs *fregs, void *entry_data); - void exit_callback(struct fprobe *fp, unsigned long entry_ip, unsigned long ret_ip, struct pt_regs *regs, void *entry_data); + void exit_callback(struct fprobe *fp, unsigned long entry_ip, unsigned long ret_ip, struct ftrace_regs *fregs, void *entry_data); -Note that the @entry_ip is saved at function entry and passed to exit handler. -If the entry callback function returns !0, the corresponding exit callback will be cancelled. +Note that the @entry_ip is saved at function entry and passed to exit +handler. +If the entry callback function returns !0, the corresponding exit callback +will be cancelled. @fp This is the address of `fprobe` data structure related to this handler. @@ -112,12 +115,10 @@ If the entry callback function returns !0, the corresponding exit callback will This is the return address that the traced function will return to, somewhere in the caller. This can be used at both entry and exit. -@regs - This is the `pt_regs` data structure at the entry and exit. Note that - the instruction pointer of @regs may be different from the @entry_ip - in the entry_handler. If you need traced instruction pointer, you need - to use @entry_ip. On the other hand, in the exit_handler, the instruction - pointer of @regs is set to the current return address. +@fregs + This is the `ftrace_regs` data structure at the entry and exit. This + includes the function parameters, or the return values. So user can + access thos values via appropriate `ftrace_regs_*` APIs. @entry_data This is a local storage to share the data between entry and exit handlers. @@ -125,6 +126,17 @@ If the entry callback function returns !0, the corresponding exit callback will and `entry_data_size` field when registering the fprobe, the storage is allocated and passed to both `entry_handler` and `exit_handler`. +Entry data size and exit handlers on the same function +====================================================== + +Since the entry data is passed via per-task stack and it has limited size, +the entry data size per probe is limited to `15 * sizeof(long)`. You also need +to take care that the different fprobes are probing on the same function, this +limit becomes smaller. The entry data size is aligned to `sizeof(long)` and +each fprobe which has exit handler uses a `sizeof(long)` space on the stack, +you should keep the number of fprobes on the same function as small as +possible. + Share the callbacks with kprobes ================================ @@ -165,8 +177,8 @@ This counter counts up when; - fprobe fails to take ftrace_recursion lock. This usually means that a function which is traced by other ftrace users is called from the entry_handler. - - fprobe fails to setup the function exit because of the shortage of rethook - (the shadow stack for hooking the function return.) + - fprobe fails to setup the function exit because of failing to allocate the + data buffer from the per-task shadow stack. The `fprobe::nmissed` field counts up in both cases. Therefore, the former skips both of entry and exit callback and the latter skips the exit