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When the trace_pid_list was created, the default pid max was 32768. Creating a bitmask that can hold one bit for all 32768 took up 4096 (one page). Having a one page bitmask was not much of a problem, and that was used for mapping pids. But today, systems are bigger and can run more tasks, and now the default pid_max is usually set to 4194304. Which means to handle that many pids requires 524288 bytes. Worse yet, the pid_max can be set to 2^30 (1073741824 or 1G) which would take 134217728 (128M) of memory to store this array. Since the pid_list array is very sparsely populated, it is a huge waste of memory to store all possible bits for each pid when most will not be set. Instead, use a page table scheme to store the array, and allow this to handle up to 30 bit pids. The pid_mask will start out with 256 entries for the first 8 MSB bits. This will cost 1K for 32 bit architectures and 2K for 64 bit. Each of these will have a 256 array to store the next 8 bits of the pid (another 1 or 2K). These will hold an 2K byte bitmask (which will cover the LSB 14 bits or 16384 pids). When the trace_pid_list is allocated, it will have the 1/2K upper bits allocated, and then it will allocate a cache for the next upper chunks and the lower chunks (default 6 of each). Then when a bit is "set", these chunks will be pulled from the free list and added to the array. If the free list gets down to a lever (default 2), it will trigger an irqwork that will refill the cache back up. On clearing a bit, if the clear causes the bitmask to be zero, that chunk will then be placed back into the free cache for later use, keeping the need to allocate more down to a minimum. Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
89 lines
3.0 KiB
C
89 lines
3.0 KiB
C
// SPDX-License-Identifier: GPL-2.0
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/* Do not include this file directly. */
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#ifndef _TRACE_INTERNAL_PID_LIST_H
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#define _TRACE_INTERNAL_PID_LIST_H
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/*
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* In order to keep track of what pids to trace, a tree is created much
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* like page tables are used. This creates a sparse bit map, where
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* the tree is filled in when needed. A PID is at most 30 bits (see
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* linux/thread.h), and is broken up into 3 sections based on the bit map
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* of the bits. The 8 MSB is the "upper1" section. The next 8 MSB is the
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* "upper2" section and the 14 LSB is the "lower" section.
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*
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* A trace_pid_list structure holds the "upper1" section, in an
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* array of 256 pointers (1 or 2K in size) to "upper_chunk" unions, where
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* each has an array of 256 pointers (1 or 2K in size) to the "lower_chunk"
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* structures, where each has an array of size 2K bytes representing a bitmask
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* of the 14 LSB of the PID (256 * 8 = 2048)
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*
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* When a trace_pid_list is allocated, it includes the 256 pointer array
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* of the upper1 unions. Then a "cache" of upper and lower is allocated
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* where these will be assigned as needed.
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*
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* When a bit is set in the pid_list bitmask, the pid to use has
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* the 8 MSB masked, and this is used to index the array in the
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* pid_list to find the next upper union. If the element is NULL,
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* then one is retrieved from the upper_list cache. If none is
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* available, then -ENOMEM is returned.
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*
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* The next 8 MSB is used to index into the "upper2" section. If this
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* element is NULL, then it is retrieved from the lower_list cache.
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* Again, if one is not available -ENOMEM is returned.
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*
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* Finally the 14 LSB of the PID is used to set the bit in the 16384
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* bitmask (made up of 2K bytes).
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*
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* When the second upper section or the lower section has their last
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* bit cleared, they are added back to the free list to be reused
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* when needed.
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*/
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#define UPPER_BITS 8
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#define UPPER_MAX (1 << UPPER_BITS)
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#define UPPER1_SIZE (1 << UPPER_BITS)
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#define UPPER2_SIZE (1 << UPPER_BITS)
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#define LOWER_BITS 14
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#define LOWER_MAX (1 << LOWER_BITS)
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#define LOWER_SIZE (LOWER_MAX / BITS_PER_LONG)
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#define UPPER1_SHIFT (LOWER_BITS + UPPER_BITS)
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#define UPPER2_SHIFT LOWER_BITS
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#define LOWER_MASK (LOWER_MAX - 1)
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#define UPPER_MASK (UPPER_MAX - 1)
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/* According to linux/thread.h pids can not be bigger than or equal to 1 << 30 */
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#define MAX_PID (1 << 30)
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/* Just keep 6 chunks of both upper and lower in the cache on alloc */
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#define CHUNK_ALLOC 6
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/* Have 2 chunks free, trigger a refill of the cache */
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#define CHUNK_REALLOC 2
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union lower_chunk {
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union lower_chunk *next;
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unsigned long data[LOWER_SIZE]; // 2K in size
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};
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union upper_chunk {
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union upper_chunk *next;
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union lower_chunk *data[UPPER2_SIZE]; // 1 or 2K in size
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};
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struct trace_pid_list {
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raw_spinlock_t lock;
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struct irq_work refill_irqwork;
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union upper_chunk *upper[UPPER1_SIZE]; // 1 or 2K in size
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union upper_chunk *upper_list;
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union lower_chunk *lower_list;
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int free_upper_chunks;
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int free_lower_chunks;
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};
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#endif /* _TRACE_INTERNAL_PID_LIST_H */
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