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Date: Wed, 8 Apr 2009 16:42:27 +0530
From: "K.Prasad" <prasad@...ux.vnet.ibm.com>
To: Frederic Weisbecker <fweisbec@...il.com>
Cc: Alan Stern <stern@...land.harvard.edu>,
Ingo Molnar <mingo@...e.hu>,
Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
Andrew Morton <akpm@...ux-foundation.org>,
Benjamin Herrenschmidt <benh@....ibm.com>,
maneesh@...ux.vnet.ibm.com, Roland McGrath <roland@...hat.com>,
Steven Rostedt <rostedt@...dmis.org>,
Steven Rostedt <srostedt@...hat.com>
Subject: Re: [Patch 11/11] ftrace plugin for kernel symbol tracing using HW
Breakpoint interfaces - v2
On Wed, Apr 08, 2009 at 10:02:03AM +0200, Frederic Weisbecker wrote:
> Hi Prasad,
>
>
> On Tue, Apr 07, 2009 at 12:07:14PM +0530, K.Prasad wrote:
> > This patch adds an ftrace plugin to detect and profile memory access over kernel
> > variables. It uses HW Breakpoint interfaces to 'watch memory addresses.
> >
> > Signed-off-by: K.Prasad <prasad@...ux.vnet.ibm.com>
> > Acked-by: Steven Rostedt <srostedt@...hat.com>
> > ---
> > kernel/trace/Kconfig | 21 +
> > kernel/trace/Makefile | 1
> > kernel/trace/trace.h | 25 +
> > kernel/trace/trace_ksym.c | 558 ++++++++++++++++++++++++++++++++++++++++++
> > kernel/trace/trace_selftest.c | 36 ++
> > 5 files changed, 641 insertions(+)
> >
> > Index: kernel/trace/Kconfig
> > ===================================================================
> > --- kernel/trace/Kconfig.orig
> > +++ kernel/trace/Kconfig
> > @@ -268,6 +268,27 @@ config POWER_TRACER
> > power management decisions, specifically the C-state and P-state
> > behavior.
> >
> > +config KSYM_TRACER
> > + bool "Trace read and write access on kernel memory locations"
> > + depends on HAVE_HW_BREAKPOINT
> > + select TRACING
> > + help
> > + This tracer helps find read and write operations on any given kernel
> > + symbol i.e. /proc/kallsyms.
> > +
> > +config PROFILE_KSYM_TRACER
> > + bool "Profile all kernel memory accesses on 'watched' variables"
> > + depends on KSYM_TRACER
> > + help
> > + This tracer profiles kernel accesses on variables watched through the
> > + ksym tracer ftrace plugin. Depending upon the hardware, all read
> > + and write operations on kernel variables can be monitored for
> > + accesses.
> > +
> > + The results will be displayed in:
> > + /debugfs/tracing/profile_ksym
> > +
> > + Say N if unsure.
> >
> > config STACK_TRACER
> > bool "Trace max stack"
> > Index: kernel/trace/Makefile
> > ===================================================================
> > --- kernel/trace/Makefile.orig
> > +++ kernel/trace/Makefile
> > @@ -46,5 +46,6 @@ obj-$(CONFIG_EVENT_TRACER) += trace_expo
> > obj-$(CONFIG_FTRACE_SYSCALLS) += trace_syscalls.o
> > obj-$(CONFIG_EVENT_PROFILE) += trace_event_profile.o
> > obj-$(CONFIG_EVENT_TRACER) += trace_events_filter.o
> > +obj-$(CONFIG_KSYM_TRACER) += trace_ksym.o
> >
> > libftrace-y := ftrace.o
> > Index: kernel/trace/trace.h
> > ===================================================================
> > --- kernel/trace/trace.h.orig
> > +++ kernel/trace/trace.h
> > @@ -12,6 +12,10 @@
> > #include <trace/kmemtrace.h>
> > #include <trace/power.h>
> >
> > +#ifdef CONFIG_KSYM_TRACER
> > +#include <asm/hw_breakpoint.h>
> > +#endif
> > +
> > enum trace_type {
> > __TRACE_FIRST_TYPE = 0,
> >
> > @@ -37,6 +41,7 @@ enum trace_type {
> > TRACE_KMEM_FREE,
> > TRACE_POWER,
> > TRACE_BLK,
> > + TRACE_KSYM,
> >
> > __TRACE_LAST_TYPE,
> > };
> > @@ -212,6 +217,23 @@ struct syscall_trace_exit {
> > unsigned long ret;
> > };
> >
> > +#ifdef CONFIG_KSYM_TRACER
> > +struct trace_ksym {
> > + struct trace_entry ent;
> > + struct hw_breakpoint *ksym_hbp;
> > + unsigned long ksym_addr;
> > + unsigned long ip;
> > +#ifdef CONFIG_PROFILE_KSYM_TRACER
> > + unsigned long counter;
> > +#endif
> > + struct hlist_node ksym_hlist;
> > + char ksym_name[KSYM_NAME_LEN];
> > + char p_name[TASK_COMM_LEN];
> > +};
> > +#else
> > +struct trace_ksym {
> > +};
> > +#endif /* CONFIG_KSYM_TRACER */
> > /*
>
>
> Is this #ifdef CONFIG_KSYM_TRACER necessary?
> On the off-case it wouldn't hurt I guess, neither
> would it fill any irrelevant space.
>
>
>
I agree. The other structures used various plugins are also defined
unconditionally. I will remove them when submitting the patch again for
inclusion.
> > * trace_flag_type is an enumeration that holds different
> > @@ -330,6 +352,7 @@ extern void __ftrace_bad_type(void);
> > TRACE_SYSCALL_ENTER); \
> > IF_ASSIGN(var, ent, struct syscall_trace_exit, \
> > TRACE_SYSCALL_EXIT); \
> > + IF_ASSIGN(var, ent, struct trace_ksym, TRACE_KSYM); \
> > __ftrace_bad_type(); \
> > } while (0)
> >
> > @@ -593,6 +616,8 @@ extern int trace_selftest_startup_syspro
> > struct trace_array *tr);
> > extern int trace_selftest_startup_branch(struct tracer *trace,
> > struct trace_array *tr);
> > +extern int trace_selftest_startup_ksym(struct tracer *trace,
> > + struct trace_array *tr);
> > #endif /* CONFIG_FTRACE_STARTUP_TEST */
> >
> > extern void *head_page(struct trace_array_cpu *data);
> > Index: kernel/trace/trace_ksym.c
> > ===================================================================
> > --- /dev/null
> > +++ kernel/trace/trace_ksym.c
> > @@ -0,0 +1,558 @@
> > +/*
> > + * trace_ksym.c - Kernel Symbol Tracer
> > + *
> > + * This program is free software; you can redistribute it and/or modify
> > + * it under the terms of the GNU General Public License as published by
> > + * the Free Software Foundation; either version 2 of the License, or
> > + * (at your option) any later version.
> > + *
> > + * This program is distributed in the hope that it will be useful,
> > + * but WITHOUT ANY WARRANTY; without even the implied warranty of
> > + * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
> > + * GNU General Public License for more details.
> > + *
> > + * You should have received a copy of the GNU General Public License
> > + * along with this program; if not, write to the Free Software
> > + * Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA 02111-1307, USA.
> > + *
> > + * Copyright (C) IBM Corporation, 2009
> > + */
> > +
> > +#include <linux/kallsyms.h>
> > +#include <linux/uaccess.h>
> > +#include <linux/debugfs.h>
> > +#include <linux/ftrace.h>
> > +#include <linux/module.h>
> > +#include <linux/jhash.h>
> > +#include <linux/fs.h>
> > +
> > +#include "trace_output.h"
> > +#include "trace_stat.h"
> > +#include "trace.h"
> > +
> > +/* For now, let us restrict the no. of symbols traced simultaneously to number
> > + * of available hardware breakpoint registers.
> > + */
> > +#define KSYM_TRACER_MAX HB_NUM
> > +
> > +#define KSYM_TRACER_OP_LEN 3 /* rw- */
> > +#define KSYM_FILTER_ENTRY_LEN (KSYM_NAME_LEN + KSYM_TRACER_OP_LEN + 1)
> > +
> > +#ifdef CONFIG_FTRACE_SELFTEST
> > +
> > +static int ksym_selftest_dummy;
> > +#define KSYM_SELFTEST_ENTRY "ksym_selftest_dummy"
> > +
> > +#endif /* CONFIG_FTRACE_SELFTEST */
> > +
> > +static struct trace_array *ksym_trace_array;
> > +
> > +DEFINE_MUTEX(ksym_tracer_mutex);
> > +
> > +static unsigned int ksym_filter_entry_count;
> > +static unsigned int ksym_tracing_enabled;
> > +
> > +static HLIST_HEAD(ksym_filter_head);
> > +
> > +#ifdef CONFIG_PROFILE_KSYM_TRACER
> > +
> > +#define MAX_UL_INT 0xffffffff
> > +DEFINE_SPINLOCK(ksym_stat_lock);
> > +
> > +void ksym_collect_stats(unsigned long hbp_hit_addr)
> > +{
> > + struct hlist_node *node;
> > + struct trace_ksym *entry;
> > +
> > + spin_lock(&ksym_stat_lock);
>
>
>
> Does this spinlock protect your list iteration against concurrent removal
> or inserts? Other readers and writers of this list use ksym_tracer_mutex
> to synchronize, it looks like this site override this rule.
>
>
>
While the ksym_stat_lock spinlock was meant to protect the counter
updation, you've unearthed the fact that the hlist whose head is
ksym_filter_head needs to be protected from simultaneous read/write
operations that can happen through ksym_trace_filter_write().
Since the same hlist is updated/read from both exception context and
otherwise (in the control plane of ftrace), ksym_stat_lock spinlock
will be used universally after replacing the ksym_tracer_mutex (this
could potentially be treated with RCU too, but choosing spinlock for
now).
I will send out another version of this patch that includes this change,
and would accept your acknowledgement. Thanks for pointing out the
potential issue.
-- K.Prasad
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