linux-kernel - Re: [PATCH] tracing/x86/xen: Remove zero data size trace events trace_xen_mmu_flush_tlb{

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Date:   Thu, 10 May 2018 10:37:22 -0400
From:   Steven Rostedt <rostedt@...dmis.org>
To:     LKML <linux-kernel@...r.kernel.org>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Ingo Molnar <mingo@...nel.org>,
        "H. Peter Anvin" <hpa@...or.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Jingjie Jiang <jingjie.jiang@...cle.com>,
        Mukesh Rathor <mukesh.rathor@...cle.com>,
        Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
        stable@...r.kernel.org
Subject: Re: [PATCH] tracing/x86/xen: Remove zero data size trace events
 trace_xen_mmu_flush_tlb{_all}

Thomas, Ingo or Peter.

Any of you willing to take this patch, or send me an ack and I'll take
it and push it to Linus?

-- Steve


On Wed, 9 May 2018 14:46:05 -0400
Steven Rostedt <rostedt@...dmis.org> wrote:

> From: "Steven Rostedt (VMware)" <rostedt@...dmis.org>
> 
> Doing an audit of trace events, I discovered two trace events in the xen
> subsystem that use a hack to create zero data size trace events. This is not
> what trace events are for. Trace events add memory footprint overhead, and
> if all you need to do is see if a function is hit or not, simply make that
> function noinline and use function tracer filtering.
> 
> Worse yet, the hack used was:
> 
>  __array(char, x, 0)
> 
> Which creates a static string of zero in length. There's assumptions about
> such constructs in ftrace that this is a dynamic string that is nul
> terminated. This is not the case with these tracepoints and can cause
> problems in various parts of ftrace.
> 
> Nuke the trace events!
> 
> Cc: stable@...r.kernel.org
> Fixes: 95a7d76897c1e ("xen/mmu: Use Xen specific TLB flush instead of the generic one.")
> Signed-off-by: Steven Rostedt (VMware) <rostedt@...dmis.org>
> ---
>  arch/x86/xen/mmu.c         |  4 +---
>  arch/x86/xen/mmu_pv.c      |  4 +---
>  include/trace/events/xen.h | 16 ----------------
>  3 files changed, 2 insertions(+), 22 deletions(-)
> 
> diff --git a/arch/x86/xen/mmu.c b/arch/x86/xen/mmu.c
> index d33e7dbe3129..2d76106788a3 100644
> --- a/arch/x86/xen/mmu.c
> +++ b/arch/x86/xen/mmu.c
> @@ -42,13 +42,11 @@ xmaddr_t arbitrary_virt_to_machine(void *vaddr)
>  }
>  EXPORT_SYMBOL_GPL(arbitrary_virt_to_machine);
>  
> -static void xen_flush_tlb_all(void)
> +static noinline void xen_flush_tlb_all(void)
>  {
>  	struct mmuext_op *op;
>  	struct multicall_space mcs;
>  
> -	trace_xen_mmu_flush_tlb_all(0);
> -
>  	preempt_disable();
>  
>  	mcs = xen_mc_entry(sizeof(*op));
> diff --git a/arch/x86/xen/mmu_pv.c b/arch/x86/xen/mmu_pv.c
> index 486c0a34d00b..2c30cabfda90 100644
> --- a/arch/x86/xen/mmu_pv.c
> +++ b/arch/x86/xen/mmu_pv.c
> @@ -1310,13 +1310,11 @@ unsigned long xen_read_cr2_direct(void)
>  	return this_cpu_read(xen_vcpu_info.arch.cr2);
>  }
>  
> -static void xen_flush_tlb(void)
> +static noinline void xen_flush_tlb(void)
>  {
>  	struct mmuext_op *op;
>  	struct multicall_space mcs;
>  
> -	trace_xen_mmu_flush_tlb(0);
> -
>  	preempt_disable();
>  
>  	mcs = xen_mc_entry(sizeof(*op));
> diff --git a/include/trace/events/xen.h b/include/trace/events/xen.h
> index 7dd8f34c37df..fdcf88bcf0ea 100644
> --- a/include/trace/events/xen.h
> +++ b/include/trace/events/xen.h
> @@ -352,22 +352,6 @@ DECLARE_EVENT_CLASS(xen_mmu_pgd,
>  DEFINE_XEN_MMU_PGD_EVENT(xen_mmu_pgd_pin);
>  DEFINE_XEN_MMU_PGD_EVENT(xen_mmu_pgd_unpin);
>  
> -TRACE_EVENT(xen_mmu_flush_tlb_all,
> -	    TP_PROTO(int x),
> -	    TP_ARGS(x),
> -	    TP_STRUCT__entry(__array(char, x, 0)),
> -	    TP_fast_assign((void)x),
> -	    TP_printk("%s", "")
> -	);
> -
> -TRACE_EVENT(xen_mmu_flush_tlb,
> -	    TP_PROTO(int x),
> -	    TP_ARGS(x),
> -	    TP_STRUCT__entry(__array(char, x, 0)),
> -	    TP_fast_assign((void)x),
> -	    TP_printk("%s", "")
> -	);
> -
>  TRACE_EVENT(xen_mmu_flush_tlb_one_user,
>  	    TP_PROTO(unsigned long addr),
>  	    TP_ARGS(addr),