old-cross-binutils/gdb/linux-nat.h
Pedro Alves 774113b02f [Linux] Optimize PID -> struct lwp_info lookup
Hacking the gdb.threads/attach-many-short-lived-threads.exp test to
spawn thousands of threads instead of dozens, and running gdb under
perf, I saw that GDB was spending most of the time in find_lwp_pid:

   - captured_main
      - 93.61% catch_command_errors
         - 87.41% attach_command
            - 87.40% linux_nat_attach
               - 87.40% linux_proc_attach_tgid_threads
                  - 82.38% attach_proc_task_lwp_callback
                     - 81.01% find_lwp_pid
                          5.30% ptid_get_lwp
                        + 0.10% ptid_lwp_p
                     + 0.64% add_thread
                     + 0.26% set_running
                     + 0.24% set_executing
                       0.12% ptid_get_lwp
                     + 0.01% ptrace
                     + 0.01% add_lwp

attach_proc_task_lwp_callback is called once for each LWP that we
attach to, found by listing the /proc/PID/task/ directory.  In turn,
attach_proc_task_lwp_callback calls find_lwp_pid to check whether the
LWP we're about to try to attach to is already known.  Since
find_lwp_pid does a linear walk over the whole LWP list, this becomes
quadratic.  We do the /proc/PID/task/ listing until we get two
iterations in a row where we found no new threads.  So the second and
following times we walk the /proc/PID/task/ dir, we're going to take
an even worse find_lwp_pid hit.

Fix this by adding a hash table keyed by LWP PID, for fast lookup.

The linked list embedded in the LWP structure itself is kept, and made
a double-linked list, so that removals from that list are O(1).  An
earlier version of this patch got rid of this list altogether, but
that revealed hidden dependencies / assumptions on how the list is
sorted.  For example, killing a process and then waiting for all the
LWPs status using iterate_over_lwps only works as is because the
leader LWP is always last in the list.  So I thought it better to take
an incremental approach and make this patch concern itself _only_ with
the PID lookup optimization.

gdb/ChangeLog:
2016-05-24  Pedro Alves  <palves@redhat.com>

	PR gdb/19828
	* linux-nat.c (lwp_lwpid_htab): New htab.
	(lwp_info_hash, lwp_lwpid_htab_eq, lwp_lwpid_htab_create)
	(lwp_lwpid_htab_add_lwp): New functions.
	(lwp_list): Tweak comment.
	(lwp_list_add, lwp_list_remove, lwp_lwpid_htab_remove_pid): New
	functions.
	(purge_lwp_list): Rewrite, using htab_traverse_noresize.
	(add_initial_lwp): Add lwp to htab too.  Use lwp_list_add.
	(delete_lwp): Use lwp_list_remove.  Remove htab too.
	(find_lwp_pid): Search in htab.
	(_initialize_linux_nat): Call lwp_lwpid_htab_create.
	* linux-nat.h (struct lwp_info) <prev>: New field.
2016-05-24 14:50:37 +01:00

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C

/* Native debugging support for GNU/Linux (LWP layer).
Copyright (C) 2000-2016 Free Software Foundation, Inc.
This file is part of GDB.
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 3 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, see <http://www.gnu.org/licenses/>. */
#include "nat/linux-nat.h"
#include "target.h"
#include <signal.h>
struct arch_lwp_info;
/* Structure describing an LWP. This is public only for the purposes
of ALL_LWPS; target-specific code should generally not access it
directly. */
struct lwp_info
{
/* The process id of the LWP. This is a combination of the LWP id
and overall process id. */
ptid_t ptid;
/* If this flag is set, we need to set the event request flags the
next time we see this LWP stop. */
int must_set_ptrace_flags;
/* Non-zero if we sent this LWP a SIGSTOP (but the LWP didn't report
it back yet). */
int signalled;
/* Non-zero if this LWP is stopped. */
int stopped;
/* Non-zero if this LWP will be/has been resumed. Note that an LWP
can be marked both as stopped and resumed at the same time. This
happens if we try to resume an LWP that has a wait status
pending. We shouldn't let the LWP run until that wait status has
been processed, but we should not report that wait status if GDB
didn't try to let the LWP run. */
int resumed;
/* The last resume GDB requested on this thread. */
enum resume_kind last_resume_kind;
/* If non-zero, a pending wait status. */
int status;
/* When 'stopped' is set, this is where the lwp last stopped, with
decr_pc_after_break already accounted for. If the LWP is
running, and stepping, this is the address at which the lwp was
resumed (that is, it's the previous stop PC). If the LWP is
running and not stepping, this is 0. */
CORE_ADDR stop_pc;
/* Non-zero if we were stepping this LWP. */
int step;
/* The reason the LWP last stopped, if we need to track it
(breakpoint, watchpoint, etc.) */
enum target_stop_reason stop_reason;
/* On architectures where it is possible to know the data address of
a triggered watchpoint, STOPPED_DATA_ADDRESS_P is non-zero, and
STOPPED_DATA_ADDRESS contains such data address. Otherwise,
STOPPED_DATA_ADDRESS_P is false, and STOPPED_DATA_ADDRESS is
undefined. Only valid if STOPPED_BY_WATCHPOINT is true. */
int stopped_data_address_p;
CORE_ADDR stopped_data_address;
/* Non-zero if we expect a duplicated SIGINT. */
int ignore_sigint;
/* If WAITSTATUS->KIND != TARGET_WAITKIND_SPURIOUS, the waitstatus
for this LWP's last event. This may correspond to STATUS above,
or to a local variable in lin_lwp_wait. */
struct target_waitstatus waitstatus;
/* Signal whether we are in a SYSCALL_ENTRY or
in a SYSCALL_RETURN event.
Values:
- TARGET_WAITKIND_SYSCALL_ENTRY
- TARGET_WAITKIND_SYSCALL_RETURN */
enum target_waitkind syscall_state;
/* The processor core this LWP was last seen on. */
int core;
/* Arch-specific additions. */
struct arch_lwp_info *arch_private;
/* Previous and next pointers in doubly-linked list of known LWPs,
sorted by reverse creation order. */
struct lwp_info *prev;
struct lwp_info *next;
};
/* The global list of LWPs, for ALL_LWPS. Unlike the threads list,
there is always at least one LWP on the list while the GNU/Linux
native target is active. */
extern struct lwp_info *lwp_list;
/* Does the current host support PTRACE_GETREGSET? */
extern enum tribool have_ptrace_getregset;
/* Iterate over each active thread (light-weight process). */
#define ALL_LWPS(LP) \
for ((LP) = lwp_list; \
(LP) != NULL; \
(LP) = (LP)->next)
/* Attempt to initialize libthread_db. */
void check_for_thread_db (void);
/* Called from the LWP layer to inform the thread_db layer that PARENT
spawned CHILD. Both LWPs are currently stopped. This function
does whatever is required to have the child LWP under the
thread_db's control --- e.g., enabling event reporting. Returns
true on success, false if the process isn't using libpthread. */
extern int thread_db_notice_clone (ptid_t parent, ptid_t child);
/* Return the set of signals used by the threads library. */
extern void lin_thread_get_thread_signals (sigset_t *mask);
/* Find process PID's pending signal set from /proc/pid/status. */
void linux_proc_pending_signals (int pid, sigset_t *pending,
sigset_t *blocked, sigset_t *ignored);
/* For linux_stop_lwp see nat/linux-nat.h. */
/* Stop all LWPs, synchronously. (Any events that trigger while LWPs
are being stopped are left pending.) */
extern void linux_stop_and_wait_all_lwps (void);
/* Set resumed LWPs running again, as they were before being stopped
with linux_stop_and_wait_all_lwps. (LWPS with pending events are
left stopped.) */
extern void linux_unstop_all_lwps (void);
/* Create a prototype generic GNU/Linux target. The client can
override it with local methods. */
struct target_ops * linux_target (void);
/* Create a generic GNU/Linux target using traditional
ptrace register access. */
struct target_ops *
linux_trad_target (CORE_ADDR (*register_u_offset)(struct gdbarch *, int, int));
/* Register the customized GNU/Linux target. This should be used
instead of calling add_target directly. */
void linux_nat_add_target (struct target_ops *);
/* Register a method to call whenever a new thread is attached. */
void linux_nat_set_new_thread (struct target_ops *, void (*) (struct lwp_info *));
/* Register a method to call whenever a new fork is attached. */
typedef void (linux_nat_new_fork_ftype) (struct lwp_info *parent,
pid_t child_pid);
void linux_nat_set_new_fork (struct target_ops *ops,
linux_nat_new_fork_ftype *fn);
/* Register a method to call whenever a process is killed or
detached. */
typedef void (linux_nat_forget_process_ftype) (pid_t pid);
void linux_nat_set_forget_process (struct target_ops *ops,
linux_nat_forget_process_ftype *fn);
/* Call the method registered with the function above. PID is the
process to forget about. */
void linux_nat_forget_process (pid_t pid);
/* Register a method that converts a siginfo object between the layout
that ptrace returns, and the layout in the architecture of the
inferior. */
void linux_nat_set_siginfo_fixup (struct target_ops *,
int (*) (siginfo_t *,
gdb_byte *,
int));
/* Register a method to call prior to resuming a thread. */
void linux_nat_set_prepare_to_resume (struct target_ops *,
void (*) (struct lwp_info *));
/* Update linux-nat internal state when changing from one fork
to another. */
void linux_nat_switch_fork (ptid_t new_ptid);
/* Store the saved siginfo associated with PTID in *SIGINFO.
Return 1 if it was retrieved successfully, 0 otherwise (*SIGINFO is
uninitialized in such case). */
int linux_nat_get_siginfo (ptid_t ptid, siginfo_t *siginfo);
/* Set alternative SIGTRAP-like events recognizer. */
void linux_nat_set_status_is_event (struct target_ops *t,
int (*status_is_event) (int status));