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/*
* Copyright (C) 2006 Michael Brown <mbrown@fensystems.co.uk>.
*
* 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 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., 675 Mass Ave, Cambridge, MA 02139, USA.
*/
#include <stddef.h>
#include <latch.h>
#include <gpxe/list.h>
#include <gpxe/process.h>
#include <gpxe/init.h>
#include <gpxe/retry.h>
/** @file
*
* Retry timers
*
* A retry timer is a binary exponential backoff timer. It can be
* used to build automatic retransmission into network protocols.
*/
/** Default timeout value */
#define MIN_TIMEOUT ( TICKS_PER_SEC / 4 )
/** Limit after which the timeout will be deemed permanent */
#define MAX_TIMEOUT ( 10 * TICKS_PER_SEC )
/* The theoretical minimum that the algorithm in stop_timer() can
* adjust the timeout back down to is seven ticks, so set the minimum
* timeout to at least that value for the sake of consistency.
*/
#if MIN_TIMEOUT < 7
#undef MIN_TIMEOUT
#define MIN_TIMEOUT 7
#endif
/** List of running timers */
static LIST_HEAD ( timers );
/**
* Start timer
*
* @v timer Retry timer
*
* This starts the timer running with the current timeout value. If
* stop_timer() is not called before the timer expires, the timer will
* be stopped and the timer's callback function will be called.
*/
void start_timer ( struct retry_timer *timer ) {
list_add ( &timer->list, &timers );
timer->start = currticks();
if ( timer->timeout < MIN_TIMEOUT )
timer->timeout = MIN_TIMEOUT;
}
/**
* Stop timer
*
* @v timer Retry timer
*
* This stops the timer and updates the timer's timeout value.
*/
void stop_timer ( struct retry_timer *timer ) {
unsigned long old_timeout = timer->timeout;
unsigned long runtime;
list_del ( &timer->list );
runtime = currticks() - timer->start;
/* Update timer. Variables are:
*
* r = round-trip time estimate (i.e. runtime)
* t = timeout value (i.e. timer->timeout)
* s = smoothed round-trip time
*
* By choice, we set t = 4s, i.e. allow for four times the
* normal round-trip time to pass before retransmitting.
*
* We want to smooth according to s := ( 7 s + r ) / 8
*
* Since we don't actually store s, this reduces to
* t := ( 7 t / 8 ) + ( r / 2 )
*
*/
timer->timeout -= ( timer->timeout >> 3 );
timer->timeout += ( runtime >> 1 );
if ( timer->timeout != old_timeout ) {
DBG ( "Timer updated to %dms\n",
( ( 1000 * timer->timeout ) / TICKS_PER_SEC ) );
}
}
/**
* Single-step the retry timer list
*
* @v process Retry timer process
*/
static void retry_step ( struct process *process ) {
struct retry_timer *timer;
struct retry_timer *tmp;
unsigned long now = currticks();
unsigned long used;
int fail;
list_for_each_entry_safe ( timer, tmp, &timers, list ) {
used = ( now - timer->start );
if ( used >= timer->timeout ) {
/* Stop timer without performing RTT calculations */
list_del ( &timer->list );
/* Back off the timeout value */
timer->timeout <<= 1;
if ( ( fail = ( timer->timeout > MAX_TIMEOUT ) ) )
timer->timeout = MAX_TIMEOUT;
DBG ( "Timer backed off to %dms\n",
( ( 1000 * timer->timeout ) / TICKS_PER_SEC ) );
/* Call expiry callback */
timer->expired ( timer, fail );
}
}
schedule ( process );
}
/** Retry timer process */
static struct process retry_process = {
.step = retry_step,
};
/** Initialise the retry timer module */
static void init_retry ( void ) {
schedule ( &retry_process );
}
INIT_FN ( INIT_PROCESS, init_retry, NULL, NULL );
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