プロセスとコンテキストスイッチ
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プロセスディスクリプタなどのプロセスのサブシステムとコンテキストスイッチのお話。
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プロセスとコンテキストスイッチ
- 6. // include/linux/sched.h struct task_struct { volatile long state; /* -1 unrunnable, 0 runnable, >0 stop struct thread_info *thread_info; atomic_t usage; unsigned long flags; /* per process flags, defined below * int prio, static_prio; struct list_head run_list; prio_array_t *array; struct list_head tasks; struct mm_struct *mm, *active_mm;
- 7. /* * pointers to (original) parent process, youngest child, yo * older sibling, respectively. (p->father can be replaced * p->parent->pid) */ struct task_struct *real_parent; /* real parent process (wh struct task_struct *parent; /* parent process */ /* ipc stuff */ struct thread_struct thread; /* filesystem information */ struct fs_struct *fs; /* open file information */ struct files_struct *files; /* namespace */ struct signal_struct *signal; struct sigpending pending; };
- 8. スレッド情報とカーネルスタック カーネルがプロセスに対して割り振る8KB(2Pages)の領域で、先頭にス レッド情報である thread_info 構造体、末尾にはカーネル用のスタッ クを配置する。
- 10. // include/linux/sched.h union thread_union { struct thread_info thread_info; unsigned long stack[THREAD_SIZE/sizeof(long)]; };
- 11. // include/asm-i386/thread_info.h struct thread_info { struct task_struct *task; /* main task st struct exec_domain *exec_domain; /* execution do unsigned long flags; /* low level fl unsigned long status; /* thread-synch __u32 cpu; /* current CPU __s32 preempt_count; /* 0 => preempta mm_segment_t addr_limit; /* thread addre 0-0xBFFFFFFF 0-0xFFFFFFFF */ struct restart_block restart_block; unsigned long previous_esp; /* ESP of the p of nested (I */ __u8 supervisor_stack[0]; };
- 12. 余談➀ カレントプロセスの取得方法。 #define current get_current() static inline struct task_struct * get_current(void) { return current_thread_info()->task; } #define THREAD_SIZE (8192) static inline struct thread_info *current_thread_info(void) { struct thread_info *ti; __asm__("andl %%esp,%0; " :"=r" (ti) : "0" (~(THREAD_SIZE - 1)) ); return ti; }
- 13. TSS(TaskStateSegment) タスク(プロセス)の状態を保持するセグメント。Linuxでは主にI/Oポート へのアクセス権限を保持するビットマップなどを保持する。各CPUに1 つずつ保持する。 // include/asm-i386/processor.h struct tss_struct { unsigned short back_link,__blh; unsigned long esp0; unsigned short ss0,__ss0h; unsigned long esp1; unsigned short ss1,__ss1h; /* ss1 is used to cache MSR_I unsigned long esp2; unsigned short ss2,__ss2h; unsigned long __cr3;
- 14. unsigned long eip; unsigned long eflags; unsigned long eax,ecx,edx,ebx; unsigned long esp; unsigned long ebp; unsigned long esi; unsigned long edi; unsigned short es, __esh; unsigned short cs, __csh; unsigned short ss, __ssh; unsigned short ds, __dsh; unsigned short fs, __fsh; unsigned short gs, __gsh; unsigned short ldt, __ldth; unsigned short trace, io_bitmap_base;
- 15. /* * The extra 1 is there because the CPU will access an * additional byte beyond the end of the IO permission * bitmap. The extra byte must be all 1 bits, and must * be within the limit. */ unsigned long io_bitmap[IO_BITMAP_LONGS + 1]; /* * Cache the current maximum and the last task that used the */ unsigned long io_bitmap_max; struct thread_struct *io_bitmap_owner; /* * pads the TSS to be cacheline-aligned (size is 0x100) */ unsigned long __cacheline_filler[35]; /* * .. and then another 0x100 bytes for emergency kernel stac */ unsigned long stack[64]; } __attribute__((packed));
- 16. 余談➁ IntelのTSSという形式規定。 TSSは元々タスク毎に存在し、その状態を保持するセグメント（これが 本来のTSSの在り方）。 TSSを用いてタスクスイッチが行われるOSでは far jmp (セグメント間 のジャンプ命令)の1命令のみでハードウェア的にタスクスイッチに必要 な処理が走る。 e.g. jmp 0048:00000000
- 17. 余談➁~続き~
- 20. // include/asm-i386/processor.h struct thread_struct { struct desc_struct tls_array[GDT_ENTRY_TLS_ENTRIES]; unsigned long esp0; unsigned long sysenter_cs; unsigned long eip; unsigned long esp; unsigned long fs; unsigned long gs; unsigned long debugreg[8]; /* %%db0-7 debug registers */ unsigned long cr2, trap_no, error_code; union i387_union i387; struct vm86_struct __user * vm86_info; unsigned long screen_bitmap; unsigned long v86flags, v86mask, saved_esp0; unsigned int saved_fs, saved_gs; unsigned long *io_bitmap_ptr; unsigned long io_bitmap_max; };
- 22. // kernel/sched.c /* * schedule() is the main scheduler function. */ asmlinkage void __sched schedule(void) { long *switch_count; task_t *prev, *next; runqueue_t *rq; prio_array_t *array; struct list_head *queue; unsigned long long now; unsigned long run_time; int cpu, idx; :
- 23. static inline task_t * context_switch(runqueue_t *rq, task_t *prev, task_t *next) { struct mm_struct *mm = next->mm; struct mm_struct *oldmm = prev->active_mm; if (unlikely(!mm)) { next->active_mm = oldmm; atomic_inc(&oldmm->mm_count); enter_lazy_tlb(oldmm, next); } else switch_mm(oldmm, mm, next); if (unlikely(!prev->mm)) { prev->active_mm = NULL; WARN_ON(rq->prev_mm); rq->prev_mm = oldmm; } switch_to(prev, next, prev); return prev; }
- 25. // include/asm-i386/mmu_context.h static inline void switch_mm(struct mm_struct *prev, struct mm_struct *next, struct task_struct *tsk) { : /* Re-load page tables */ load_cr3(next->pgd); : } // include/asm-i386/processor.h #define load_cr3(pgdir) asm volatile("movl %0,%%cr3": :"r" (__pa(pgdir)))
- 26. カーネルモードスタックとハードウェアコン テキストの切り替え // include/asm-i386/system.h #define switch_to(prev,next,last) do { unsigned long esi,edi; asm volatile("pushfl" "pushl %%ebp" "movl %%esp,%0" /* save ESP */ "movl %5,%%esp" /* restore ESP */ "movl $1f,%1" /* save EIP */ "pushl %6" /* restore EIP */ "jmp __switch_to" "1:" "popl %%ebp" "popfl" :"=m" (prev->thread.esp), "=m" (prev->thread.eip), "=a" (last),"=S" (esi),"=D" (edi) :"m" (next->thread.esp), "m" (next->thread.eip), "2" (prev), "d" (next)); } while (0)
- 27. // archi386kernelprocess.c struct task_struct fastcall * __switch_to(struct task_struct { struct thread_struct *prev = &prev_p->thread, *next = &next_p->thread; int cpu = smp_processor_id(); struct tss_struct *tss = &per_cpu(init_tss, cpu); : __unlazy_fpu(prev_p); : if (unlikely(prev->io_bitmap_ptr || next->io_bitmap_ptr)) handle_io_bitmap(next, tss); return prev_p; } // includeasm-i386linkage.h #define fastcall __attribute__((regparm(3)))